Emerging Two Dimensional Materials and Applications 1032162872, 9781032162874

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Emerging Two Dimensional Materials and Applications This book details 2D nanomaterials, and their important applications—including recent developments and related scalable technologies crucial to addressing strong societal demands of energy, environmental protection, and worldwide health concerns—are systematically documented. It covers syntheses and structures of various 2D materials, electrical transport in graphene, and different properties in detail. Applications in important areas of energy harvesting, energy storage, environmental monitoring, and biosensing and health care are elaborated. Features: • Facilitates good understanding of concepts of emerging 2D materials and its applications. • Covers details of highly sensitive sensors using 2D materials for environmental monitoring. • Outlines the role of 2D materials in improvement of energy harvesting and storage. • Details application in biosensing and health care for the realization of nextgeneration biotechnologies for personalized health monitoring and so forth. • Provides exclusive coverage of inorganic 2D MXenes compounds. This book is aimed at graduate students and researchers in materials science and engineering, nanoscience and nanotechnology, and electrical engineering.

Emerging Materials and Technologies Series Editor: Boris I. Kharissov Polymers Electrolytes and their Composites for Energy Storage/Conversion Devices Edited by Achchhe Lal Sharma, Anil Arya and Anurag Gaur Hybrid Polymeric Nanocomposites from Agricultural Waste Sefu Adekunle Bello Photoelectrochemical Generation of Fuels Edited by Anirban Das, Gyandshwar Kumar Rao and Kasinath Ojha Emergent Micro- and Nanomaterials for Optical, Infrared, and Terahertz Applications Edited by Song Sun, Wei Tan, and Su-Huai Wei Gas Sensors: Manufacturing, Materials, and Technologies Edited by Ankur Gupta, Mahesh Kumar, Rajeev Kumar Singh and Shantanu Bhattacharya Environmental Biotechnology: Fundamentals to Modern Techniques Sibi G Emerging Two Dimensional Materials and Applications Edited by Arun Kumar Singh, Ram Sevak Singh, and Anar Singh Advanced Porous Biomaterials for Drug Delivery Applications Edited by Mahaveer Kurkuri, Dusan Losic, U. T. Uthappa and Ho-Young Jung Thermal Transport Characteristics of Nanofuids and Phase Change Materials S. Harikrishnan and A. D. Dhass Multidimensional Lithium-Ion Battery Status Monitoring Shunli Wang, Kailong Liu, Yujie Wang, Daniel-Ioan Stroe, Carlos Fernandez, and Josep M. Guerrero For more information about this series, please visit: www.routledge.com/EmergingMaterials-and-Technologies/book-series/CRCEMT

Emerging Two Dimensional Materials and Applications

Edited by Arun Kumar Singh, Ram Sevak Singh, and Anar Singh

Boca Raton London New York

CRC Press is an imprint of the Taylor & Francis Group, an informa business

First edition published 2023 by CRC Press 6000 Broken Sound Parkway NW, Suite 300, Boca Raton, FL 33487–2742 and by CRC Press 4 Park Square, Milton Park, Abingdon, Oxon, OX14 4RN CRC Press is an imprint of Taylor & Francis Group, LLC © 2023 selection and editorial matter, Arun Kumar Singh, Ram Sevak Singh and Anar Singh; individual chapters, the contributors Reasonable efforts have been made to publish reliable data and information, but the author and publisher cannot assume responsibility for the validity of all materials or the consequences of their use. The authors and publishers have attempted to trace the copyright holders of all material reproduced in this publication and apologize to copyright holders if permission to publish in this form has not been obtained. If any copyright material has not been acknowledged please write and let us know so we may rectify in any future reprint. Except as permitted under U.S. Copyright Law, no part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microflming, and recording, or in any information storage or retrieval system, without written permission from the publishers. For permission to photocopy or use material electronically from this work, access www. copyright.com or contact the Copyright Clearance Center, Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923, 978–750–8400. For works that are not available on CCC please contact [email protected] Trademark notice: Product or corporate names may be trademarks or registered trademarks and are used only for identifcation and explanation without intent to infringe. Library of Congress Cataloging-in-Publication Data A catalog record for this book has been requested ISBN: 978-1-032-16287-4 (hbk) ISBN: 978-1-032-16288-1 (pbk) ISBN: 978-1-003-24789-0 (ebk) DOI: 10.1201/9781003247890 Typeset in Times by Apex CoVantage, LLC

Contents Editors’ Biographies ................................................................................................ vii List of Contributors................................................................................................... ix Preface.....................................................................................................................xiii Chapter 1

Overview of 2D Materials.................................................................... 1 Arun Kumar Singh, Ram Sevak Singh, and Anar Singh

Chapter 2

Synthesis and Structure of 2D Materials ............................................. 7 Bijoy Kumar Das and R. Gopalan

Chapter 3

Electrical Properties of Graphene: Comprehensive Study..................17 Sourav Sarkar, Samik Saha, and Sachindranath Das

Chapter 4

Electrical Transport in Post-Graphene 2D Materials..........................39 Ghulam Dastgeer, Sobia Nisar, and Jonghwa Eom

Chapter 5

2D Material Photonics and Optoelectronics .......................................71 Muhammad Farooq Khan, H. M. Waseem Khalil, Shania Rehman, Muhammad Asghar Khan, and Jonghwa Eom

Chapter 6

Magnetic Properties of 2D Materials................................................. 95 Rajesh Katoch and Shilpee Jain

Chapter 7

2D Materials: Mechanical Properties and Applications ...................123 Deeksha Nagpal, Astakala Anil Kumar, Jashandeep Singh, Ajay Vasishth, Shashank Priya, Ashok Kumar, and Shyam Sundar Pattnaik

Chapter 8

2D Materials for Energy Harvesting.................................................145 Astakala Anil Kumar, Sharmila Kumari Arodhiya, Shashank Priya, Ashok Kumar, and Shyam Sundar Pattnaik

Chapter 9

2D Nanomaterials for Energy Applications ......................................167 Sayli Pradhan and Neetu Jha v

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Chapter 10 Role of 2D Materials in Environmental Monitoring.........................195 Renu Dhahiya, Moumita Saha, Ashok Kumar, Pankaj Sharma, Ram Sevak Singh, Varun Rai, and Kamalakanta Behera Chapter 11 2D Nanomaterials in Diagnostics and Therapy of Cardiovascular Diseases ...............................................................217 Pooja Yadav, Samir K. Beura, Abhishek R. Panigrahi, Abhinaba Chatterjee, Jyoti Yadav, and Sunil K. Singh Index .......................................................................................................................241

Editors’ Biographies Dr. Arun Kumar Singh is working as an associate professor at Department of Pure and Applied Physics, Guru Ghasidas Vishwavidyalaya, Bilaspur (CG), India. He received his MSc degree in physics from Banaras Hindu University (BHU), Varanasi, India, and obtained his PhD degree from School of Materials Science and Technology, IIT (BHU), India. After his PhD, he joined post-doctoral research work at Graphene Research Institute, Sejong University, South Korea. He got India’s most prestigious research award, Inspire Faculty Award, from the Department of Science and Technology, India. He also visited many countries for research/collaborative work. He got many fellowships/awards from different scientifc societies of India and abroad. He has published many research papers in renowned international journals—including Advanced Functional Materials, ACS Applied Materials & Interfaces, Journal of Materials Chemistry C, Applied Materials Today, Sensors and Actuators-B, and Journal of Applied Physics—and also written a book/book chapters. His research work basically includes materials in science and engineering; organic semiconductors; nanomaterials, including carbon nanotubes, graphene, and other two-dimensional materials; electronic/optoelectronic devices; and materials for energy conversion and storage. He is a member of many scientifc societies, like American Chemical Society, Materials Research Society of India, Indian Science Congress Association, Electron Microscope Society of India, Indian Physics Association, etc. He is also a reviewer of many scientifc journals. Dr. Ram Sevak Singh is currently working as an associate professor in the Department of Physics, OP Jindal University, Raigarh, Chhattisgarth, India. He received his PhD in physics from National University of Singapore, MTech in materials science and engineering from IIT Kharagpur, and MSc in physics from Banaras Hindu University. He also served as an assistant professor in Physics Department, NIT Kurukshetra, India, and as a post-doctoral research fellow in Centre for Nano and Soft Matter Sciences, Bangalore, India, Nanyang Technological University, Singapore, and National University of Singapore. He has also received prestigious NUS Research Scholarship, Singapore, and IETE-CEOT (94) Award (Biennial)-2014, India. He is a member of American Chemical Society, Materials Research Society of India, and the Graphene Council. Dr. Singh has several years of research and teaching experiences in the areas of physics and nanotechnology and has published many research articles in journals of international repute, including ACS Nano, Nano Letters, Carbon, Renewable Energy, and Applied Physics Letters, and book chapters with vii

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Wiley and Elsevier. He is also reviewer of several reputed international journals. His areas of interest include materials physics; nanomaterials, including nanotubes, graphene, and other two-dimensional materials; optoelectronic devices; and materials for sensors, corrosion protection, energy conversion, and storage. Dr. Anar Singh earned his MSc degree in physics with specialization in solid state physics from Banaras Hindu University (BHU), Varanasi, in 2004. He received the Prof. B. Dayal Gold Medal and BHU Medal for securing the highest percentage of marks in the MSc examination. In July 2006, he joined the PhD program in the School of Materials Science and Technology (SMST), Indian Institute of Technology (BHU), Varanasi. After receiving his PhD in 2013, he joined the Paul Scherrer Institute, Switzerland, as a post-doctoral fellow. He also worked as a research fellow at National University of Singapore, Singapore, from July 2014 to June 2015. He received Inspire Faculty Award from DST, India, in 2015 and joined Institute of Chemical Technology, Mumbai, as Inspire Faculty. In June 2016, he joined the University of Lucknow as assistant professor. He has visited large-scale facility centers FRM II, Germany, and ANSTO, Australia. He has also been part of the Indo-Japanese Science Collaboration Program with Hiroshima University and Tohoku University, Japan. In 2019, he has been offered a visiting research position at University of Johannesburg, South Africa, for three years. His research interests are phase transitions in bulk and thin flms, crystallography (nuclear and magnetic structures), multiferroic materials, ferroelectrics, and piezoelectrics.

Contributors Sharmila Kumari Arodhiya Department of Physics Rajiv Gandhi Government College for Women Bhiwani, Haryana, India

Sachindranath Das Department of Instrumentation Science Jadavpur University Kolkata, India

Department of Physics National Institute of Technology Kurukshetra, Haryana, India

Ghulam Dastgeer Department of Physics and Astronomy Sejong University Seoul, Korea

Kamalakanta Behera Department of Applied Chemistry (CBFS-ASAS) Amity University Gurugram Manesar, Haryana, India

Renu Dhahiya Department of Applied Sciences National Institute of Technical Teachers Training and Research Chandigarh, India

Department of Chemistry, Faculty of Science University of Allahabad Prayagraj, India

Jonghwa Eom Department of Physics and Astronomy Sejong University Seoul, Korea

Samir K. Beura Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India

R. Gopalan Centre for Automotive Energy Materials International Advanced Research Centre for Powder Metallurgy and New Materials Taramani, Chennai, India

Abhinaba Chatterjee Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India Bijoy Kumar Das Centre for Automotive Energy Materials International Advanced Research Centre for Powder Metallurgy and New Materials Taramani, Chennai, India

Shilpee Jain Society for Innovation and Development, Innovation Centre Indian Institute of Science Bengaluru, Karnataka, India Neetu Jha Department of Physics Nathalal Parekh Marg Institute of Chemical Technology Mumbai Matunga, Mumbai

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Rajesh Katoch Institut national de la recherche scientifque Centre Énergie Matériaux et Télécommunications Varennes, Québec, Canada H. M. Waseem Khalil Department of Electrical Engineering, College of Engineering and Technology University of Sargodha Sargodha, Punjab, Pakistan Muhammad Asghar Khan Department of Physics and Astronomy Graphene Research Institute-Texas Photonics Center International Research Center (GRI–TPC IRC), Sejong University Seoul, Korea Muhammad Farooq Khan Department of Electrical Engineering Sejong University Neungdong-ro, Gwangjin-gu, Korea Ashok Kumar Department of Applied Sciences National Institute of Technical Teachers Training and Research Chandigarh, India Astakala Anil Kumar Nanomaterials for Photovoltaics and Biomaterials Laboratory Godavari Institute of Engineering and Technology Rajahmundry, India Deeksha Nagpal Department of Physics Chandigarh University Gharuan, Mohali (Punjab), India

Contributors

Sobia Nisar Department of Industrial Engineering and Management University of Engineering and Technology Taxila, Pakistan Abhishek R. Panigrahi Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India Shyam Sundar Pattnaik Media Engineering National Institute of Technical Teachers Training and Research Chandigarh, India Sayli Pradhan Department of Physics Nathalal Parekh Marg, Institute of Chemical Technology Mumbai Matunga, Mumbai Shashank Priya Materials Research Institute Penn State University State College, Pennsylvania Varun Rai Department of Chemistry National University of Singapore 3 Science Drive 3, Singapore Department of Chemistry, Faculty of Science University of Allahabad Prayagraj, India Shania Rehman Department of Electrical Engineering, College of Engineering and Technology University of Sargodha Pakistan

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Contributors

Moumita Saha Department of Chemistry Institute of Science, Banaras Hindu University Varanasi, India

Jashandeep Singh Department of Physics Gulzar Group of Institutes Khanna Khanna, Punjab, India

Samik Saha Department of Physics General Degree College Dantan-II West-Midnapore, West Bengal

Ram Sevak Singh Department of Physics OP Jindal University Raigarh, Chhattisgarh, India

Sourav Sarkar Department of Instrumentation Science Jadavpur University Kolkata, India

Sunil K. Singh Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India

Pankaj Sharma Department of Applied Sciences National Institute of Technical Teachers Training and Research Chandigarh, India

Ajay Vasishth Department of Physics Chandigarh University Gharuan, Mohali (Punjab), India

Anar Singh Department of Physics University of Lucknow Lucknow, India

Jyoti Yadav Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India

Arun Kumar Singh Department of Pure and Applied Physics Guru Ghasidas Vishwavidyalaya Bilaspur, India

Pooja Yadav Department of Zoology, School of Biological Sciences Central University of Punjab Bathinda, Punjab, India

Preface Nanomaterials are the key components of nanotechnology. Compared to their bulk counterpart, physicochemical properties of the nanomaterials dramatically changes due to the quantum confnement effect and increase in surface-to-volume ratio. Twodimensional (2D) materials are a new class of emerging nanomaterials with thickness down to atomic scale. The discovery of graphene in 2004 has stimulated tremendous research interest in fnding out other new 2D materials and their potential applications. Owing to fascinating properties, 2D materials have potential applications in various sectors, including energy conversion and storage, electronics and optoelectronics, corrosion, polymer electrolyte membrane fuel cells, memory storage, and biomedical felds. In comparison with 0D and 1D nanomaterials, the 2D nanomaterials possess a large surface area, superior electron mobility, more surface active sites, and serve as excellent photocatalytic supports and good electron transfer platforms. The book covers almost all aspects of 2D nanomaterials and their important applications. The chapters are arranged systematically to make the topic more understandable. Chapter 1 gives a brief introduction to 2D materials, covering their history of development and current practical applications. Chapter 2 comprises syntheses and structures of various 2D materials. Chapter 3 focuses on electrical transport in graphene, which has stimulated potential interests in the discovery and study of other 2D materials. Chapters 4–7 cover different properties in detail, focusing more on recent developments and applications. Chapters 8–11 shed light on more detailed applications in important areas of energy harvesting, energy storage, environmental monitoring, biosensing, and health care. The book aims to attract a large number of readers throughout the world as topics covered in the book are the most recent, emerging, and of great interest to the current scientifc community. The book also encourages solutions to many problems faced by industries. This book differs from other published books on 2D materials due to the following grounds. First, it deals with many aspects of 2D materials and their potential applications systematically to help the readers understand the topic in a better way. Second, it includes the most up-to-date published results as research in this area is being progressed day by day. Third, the chapters of the book have been written by authors having prolonged research and academic experiences from all over the world. Simplifed and illustrative fgures, tables, and language are other attractive features of this book. The book will be very helpful to scientists, engineers, and students of science and engineering backgrounds to explore the world of nanotechnology and its innovative roles for a better world in the future. Arun Kumar Singh Ram Sevak Singh Anar Singh

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Overview of 2D Materials Arun Kumar Singh, Ram Sevak Singh, and Anar Singh

CONTENTS Introduction ................................................................................................................ 1 Organization of Book................................................................................................. 3 References .................................................................................................................. 4

INTRODUCTION Two-dimensional (2D) materials are the materials consisting of a single layer of atoms. Whenever we think about two-dimensional materials, the frst material that comes to mind is graphene. Graphene is single sheet of sp2-bonded carbon atoms arranged in a honeycomb lattice.1 It is a basic building block for all other dimensionalities of carbon nanostructures. We can wrap it to form 0D fullerenes, rolled into 1D nanotubes, or stacked into 3D graphite.1–3 Before graphene, scientists argued that strictly 2D crystals were thermodynamically unstable and could not exist.4–5 In 2004, Geim and Novoselov were successfully isolated single-layer graphene and received the 2010 Nobel Prize in physics for their outstanding work. This breakthrough led to the intensive research not only by physicists and chemists but also inspired renewed interest in carbon-based electronics from device engineers and other applications.6–9 Graphene was the frst 2D material to represent a major advancement in science and technology due to its unique properties, such as electronic, mechanical, optical, and thermal property. Graphene possesses unique linear band structure, remarkable high charge carrier mobility at room temperature, high optical transmittance, fexibility in very low manufacturing cost, which makes graphene a suitable candidate for many exciting applications.10–13 Many interesting ideas have been demonstrated in the laboratory, and some of the promising applications include fexible and transparent conducting electrodes, sensors, supercapacitors, low power switches, solar cells, battery, spin devices, nanocomposites, and tunable plasmonic devices for THz and mid-infrared applications.14–17 Some more attractive products may materialize at the industrial scale in the coming years. Graphene has received widespread attention since it was frst isolated from graphite and has been demonstrated in many technological applications in nanoelectronics, optoelectronics, energy harvesting, and sensors. Graphene has a conical Dirac spectrum and linear dispersion characteristic. However, the absence of an energy gap in the electronic band structure limits its practical applications in nanoelectronics and optoelectronics. Recently the feld has rapidly expanded beyond graphene and many other 2D materials—such as the transition-metal dichalcogenides (TMDCs), transition-metal oxides and boron nitride—have been investigated.18–20 MXene, a new 2D DOI: 10.1201/9781003247890-1

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material, has been discovered more recently, which consists of a-few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides.21 The formula for TMDCs is MX2, where M is a transition metal element of group IV, V, VI, and X is a chalcogen (S, Se, or Te). These materials form layered structures of the form X-M-X, with the chalcogen atoms in two hexagonal planes separated by a plane of metal atoms.18 Many of TMDCs have fnite bandgaps, such as MoS2, WS2, MoSe2, WSe2, and many more. Bulk MoS2 is an n-type semiconductor with an indirect bandgap of 1.2 eV (where single-layer MoS2 has a direct bandgap of 1.8 eV) and has attracted increasing interest for its novel electronic and optoelectronic properties.22–24 Fascinatingly, the library of 2D layered materials grows very rapidly every day; currently, it consists of more than 150 interesting families. Research regarding 2D materials is in a growing phase, and many new materials are introduced and added to their list every year. TMDCs have many attractive properties, such as layer-dependent properties, electronic, and optical. Some of the TMDCs are metallic, some are semiconducting, and some are even superconducting.18 Another class of materials is hexagonal boron nitride (h-BN), which is an insulating 2D material.25 The layer structure h-BN is similar to graphite. It has excellent thermal and chemical stability and perfect dielectric materials for many applications. The combination of these 2D materials, we can fabricate many fascinating devices. For example, among many applications, the feld-effect transistor (FETs) is the most important and is a basic element of any electronic circuit.26 To fabricate the FETs, we require three types of materials, metal, semiconductor, and insulator. By using 2D materials, we can fabricate very thin FETs. Since graphene can be used as thinnest (thickness of single-layer graphene is 0.34 nm) and transparent electrode material, 2D TMDCs semiconductors can be used as channel materials, and h-BN can be utilized as gate dielectric, which is also very thin. Overall thickness of device would be within 2–3 nm. As such, miniaturization of electronic devices (reducing the size of devices) can be achieved more effectively using 2D materials, which dramatically improves the performance and reduces the cost of a device. Therefore, with the combination of many 2D materials, we can fabricate the high performance for fexible as well as very thin devices. It is easy to do integration and miniaturization of devices using 2D materials as compared to onedimensional or zero-dimensional nanomaterials. Even various homo and hetero junctions in both planner and vertical confguration has already been demonstrated by researchers all around the world.27 The 2D materials have unconventional and multifunctional features that will trigger further research and will hopefully overcome the constraints appeared along with the various applications shown in Figure 1.1. The ability to isolate, mix, and match highly distinct atomic layers at mono or few-atom layer thickness can facilitate thrilling opportunities to manipulate the confnement and transport of electrons, holes, photons, excitons, and phonons at the limit of single-atom thickness and to enable novel devices with extraordinary performance or entirely new functions. Although 2D materials have many exciting properties, however, it is limited with low yield synthesis and reproducibility of fabricated devices. These problems can be solved using large and wafer scale synthesis as well as device fabrication using existing semiconductor fabrication techniques.

Overview of 2D Materials

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FIGURE 1.1 Schematic diagram of various 2D materials and their applications.

ORGANIZATION OF BOOK This book originated from the need to bridge the gap of knowledge for researchers of any discipline of science or engineering when they try to start the research in the area of 2D materials. The book covers synthesis, properties, and various applications of 2D nanomaterials. The chapters are arranged systematically and divided into self-suffcient eleven chapters. Chapter 1 presents an overview of 2D materials with glimpses of various chapters in the book. Chapter 2 focuses on the different synthesis methods of 2D materials. This chapter also illustrates the structure of various 2D materials. Chapter 3 deals with the electrical property of different forms of graphene and its dependence on external parameters like temperature, pressure, feld, doping, etc. The electrical conductivity of graphene changes dramatically as it is grown from monolayer to multilayer. A discussion on effect of twisting angle between two layers, chemical potential, and impurity on the electrical properties of bilayer and multilayer graphene is also included in this chapter. Chapter 4 comprises the electrical transport and challenges of the electronic devices based on post-graphene 2D semiconductor materials. The contact engineering of metal-2D semiconductor materials is discussed in detail, which plays an essential role in charge transportation. This chapter also addresses electric properties of lateral and vertically stacked van der Waals heterostructure based on 2D semiconductor materials and highlights the role of gate-assisted electric transport through these devices. Chapter 5 deals with 2D materials and their role in the manufacturing of photonic and optoelectronic devices, such as broadband photodetectors, photovoltaic devices, optoelectronic memory, bio-photonic, light-emitting diode (LED), and MXenesbased photodetectors. Also, a great podium will be provided here to investigate the TMDCs and MXenes for their advantages, challenges, and capabilities to play an

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important role in future technology. Chapter 6 focuses on magnetic properties of 2D layered materials, including half-metals like graphene, transition metal dichalcogenides (TMDs), transition metal thiophosphates (TMPX3), transition metal halides (TMHs), and topological insulators (TIs). Also discussed is the various characterization techniques for probing magnetic properties in 2D materials and novel applications. Chapter 7 deals with the mechanical properties of 2D materials such as graphene, transition metal dichalcogenides, hexagonal boron nitride, phosphorus, and MXene, and the infuence of defects on the mechanical characteristics of 2D materials has also been discussed. Chapter 8 focuses on the characteristic attainments regarding the application of the 2D materials in the various energy-harvesting devices, such as perovskite and dye sensitized solar cells, and thermoelectric and piezoelectric devices. Chapter 9 deals with 2D materials as electrode materials in the energy storage devices. The 2D material morphology minimizes the structural and volumetric transformations associated with the intercalation/deintercalation of mobile charges associated. This chapter also discusses the different parameters of 2D materials based energy storage devices. Chapter 10 deals with the role of 2D materials in environmental monitoring, which is essential for healthy and sustainable life. In Chapter 11, the authors critically evaluate the recent advances of promising 2D nanomaterials and their characteristic features for both their use in biosensors as well as tissue engineering. We have also discussed emerging approaches for the interaction of 2D nanomaterials and their application in cardiac tissue engineering considering their associated limitations. The aim of the book is to attract and help readers from all over the world as it is of great interest to the current scientifc feld and to encourage solutions to real-world problems. The present book is signifcantly different from other published books on 2D materials as it covers almost all aspects of 2D materials and their potential applications with the inclusion of the most up-to-date published results. Importantly, the chapters of the book have been written by the best authors from all over the globe. Simplifed language with illustrative fgures and tables makes the book attractive. The book will be very helpful to scientists, engineers, and students of science and engineering backgrounds to explore the world of nanotechnology and its innovative roles for a better world in the future.

REFERENCES 1. Geim A. K. and Novoselov K. S. The rise of graphene. Nature Materials 6, 183–191 (2007). 2. Kumar P., Singh A. K., Eom J. and Singh J. Graphene: Synthesis, properties and application in Transparent Electronic Devices. Reviews in Advanced Sciences and Engineering 2, 238–258 (2013). 3. Novoselov, K. S., et al. Electric feld effect in atomically thin carbon flms. Science 306, 666–669 (2004). 4. Peierls, R. E. Quelques proprietes typiques des corpses solides. Ann. I. H. Poincare 5, 177–222 (1935). 5. Landau, L. D. Zur Theorie der phasenumwandlungen II. Physikalische Zeitschrift der Sowjetunion 11, 26–35 (1937).

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6. Singh R. S., Wang X., Ariando, Chen W. and Wee A. T. S. Large room- temperature quantum linear magnetoresistance in multilayered epitaxial graphene: Evidence for twodimensional magnetotransport. Applied Physics Letters 101, 183105 (2012). 7. Singh A. K. and Eom J. Negative magnetoresistance in vertical single layer graphene spin valve at room temperature. ACS Applied Materials & Interfaces 6, 2493−2496 (2014). 8. Singh A. K., Iqbal M. W., Singh V. K., Iqbal M. Z., Lee J. H., Chun S. H., Shin K. and Eom J. Molecular n-doping of chemical vapor deposition grown graphene. Journal of Materials Chemistry 22, 15168–15174 (2012). 9. Singh A. K., Ahmad M., Singh V. K., Shin K., Seo Y. and Eom J. Tailoring of electronic properties of exfoliated graphene layer by molecular doping. ACS Applied Materials & Interfaces 5, 5276–5281 (2013). 10. Singh R. S., Rasheed A., Gautam A., Singh A. K. and Rai V. Enhanced optical and electrical properties of graphene oxide-silver nanoparticles nanocomposite flm by thermal annealing in the air. Russian Journal of Applied Chemistry 94, 402–409 (2021). 11. Singh R. S., Gautam A. and Rai V Graphene-based bipolar plates for polymer electrolyte membrane fuel cells. Frontiers of Materials Science 13, 217 (2019). 12. Pandey R. K., Singh A. K. and Prakash R. Enhancement in performance of polycarbazole-graphene nanocomposite Schottky diode. AIP Advances 3, 122120 (2013). 13. Singh R. S., Li D., Xiong Q., Santoso I., Chen W., Rusydi A. and Wee A. T. S. Anomalous photoresponse in the deep-ultraviolet due to resonant excitonic effects in oxygen plasma treated few-layer graphene. Carbon 106, 330 (2016). 14. Singh R. S., Jansen M., Ganguly D., Kulkarni G. U., Ramaprabhu S., Choudhary S. K. and Pramanik C. Shellac derived graphene flms on solid, fexible, and porous substrates for high performance bipolar plates and supercapacitor electrodes. Renewable Energy 181, 1008 (2022). 15. Singh R. S., Nalla V., Chen W., Ji W. and Wee A. T. S. Photoresponse inepitaxial graphene with asymmetric metal contacts. Applied Physics Letters 100, 093116 (2012). 16. Singh A. K., Hwang C. and Eom J. Low-voltage and high performance multilayer MoS2 feld-effect transistors with graphene electrodes. ACS Applied Materials & Interfaces 8, 34699–34705 (2016). 17. Andleeb S., Eom J., Naz N. R. and Singh A. K. MoS2 feld effect transistor with graphene contacts. Journal of Materials Chemistry C 5, 8308 (2017). 18. Wang Q. H., Zadeh K. K., Kis A., Coleman J. N. and Strano M. S. Electronics and optoelectronics of two-dimensional transition metal dichalcogenides. Nature Nanotechnology 7, 699 (2012). 19. Singh A. K., Kumar P., Late D. J., Kumar A., Patel, S. and Singh J. 2D layered transition metal dichalcogenides (MoS2): Synthesis, applications & theoretical aspects. Applied Materials Today 13, 242–270 (2018). 20. Khan K., Tareen A. K., Aslam M., Wang R., et al. Recent developments in emerging two dimensional materials and their applications. Journal of Materials Chemistry C 8, 387 (2020). 21. Naguib M., Kurtoglu M., Presser V., Lu J., Niu J., Heon M., Hultman L., Gogotsi Y. and Barsoum M. W. Two‐dimensional nanocrystals produced by exfoliation of Ti3AlC2. Advanced Materials 23, 4248–2453 (2011). 22. Singh A. K., Andleeb S., Singh J., Dung H. T., Seo Y. and Eom J. Ultra violet light induced reversible and stable carrier modulation in MoS2 feld effect transistors. Advanced Functional Materials 24, 7125–7132 (2014). 23. Singh A. K., Pandey R. K., Prakash R. and Eom J. Tailoring the charge carrier in few layers MoS2 feld-effect transistors by Au metal adsorbate. Applied Surface Science 437, 70–74 (2018).

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24. Chaudhary V., Pandey R. K., Prakash R., Kumar N. and Singh A. K. Unfolding photophysical properties of poly(3-hexylthiophene)-MoS2 organic-inorganic hybrid materials: An application to self-powered photodetectors. Nanotechnology 32, 385201 (2021). 25. Singh R. S., Tay R. Y., Chow W. L., Tsang S. H., Mallick G. and Teo E. H. T. Band gap effects of hexagonal boron nitride using oxygen plasma. Applied Physics Letters 104, 163101 (2014). 26. Chaudhary V., Pandey R. K., Shahu P. K., Prakash R., Kumar N. and Singh A. K. MoS2 assisted self-assembled poly(3-hexylthiophene) thin flms at an air/liquid interface for high-performance feld-effect transistors under ambient conditions. The Journal of Physical Chemistry C 124, 8101−8109 (2020). 27. Jin C., Ma E. Y., Karni O., Regan E. C., Wang F. and Heinz T. F. Ultrafast dynamics in van der Waals heterostructures. Nature Nanotechnology 13, 994–1003 (2018).

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Synthesis and Structure of 2D Materials Bijoy Kumar Das and R. Gopalan

CONTENTS 2.1 2.2

General Introduction to 2D Materials .............................................................. 7 Synthesis of 2D Materials................................................................................10 2.2.1 Top-Down Approach ...........................................................................10 2.2.1.1 Mechanical Exfoliation.........................................................10 2.2.1.2 Exfoliation via Ion Intercalation ...........................................11 2.2.2 Bottom-Up ...........................................................................................11 2.2.2.1 Soft Chemical Synthesis Route.............................................11 2.2.2.2 Chemical Vapor Deposition (CVD) Route............................11 2.3 Synthesis of Transition Metal Dichalcogenides (TMDC) ...............................12 2.4 Synthesis of MXenes .......................................................................................12 2.5 Synthesis of Silicene........................................................................................13 2.6 Summary .........................................................................................................13 Acknowledgment ......................................................................................................14 References.................................................................................................................14

2.1 GENERAL INTRODUCTION TO 2D MATERIALS Currently, immense research and development (R&D) activities are focused on developing advanced materials with improved functional properties, which can be successfully integrated into devices [1–3]. The functional properties of these materials are mainly dependent on their composition, microstructure, and sizes. Developing materials at nanoscale can change the functional properties compared to their bulk [4, 5]. The reduction of size to nanoscale leads to confnement of electrons, known as quantum confnement, thereby leading to better functional properties [4, 5]. Based on the size reduction of materials to nanoscale, they can be broadly classifed into zero-dimension (0D), such as quantum dot; one-dimension (1D), such as nanowire; two-dimension (2D), such as nanosheet; and three-dimension (3D), such as bulk material (TABLE 2.1) [4, 5]. The discovery of graphene by Geim and Novoselov in 2004, which led them to receive a Nobel Prize in 2010, has raised the possibility of obtaining stable 2D materials of single-layer or multilayer through exfoliation of van der Waals solids [6]. Such 2D materials can show improved functional properties such as quantum hall effect, transport properties, and excellent optical transparency [4–7]. The successful development and implementation of graphene into device application has created plenty of interest to further develop 2D materials beyond graphene. Such interest has resulted in the DOI: 10.1201/9781003247890-2

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TABLE 2.1 Classifcations and Examples of Materials Sl. No. 1 2 3 4

Dimension

Classifcations

Examples

0D 1D 2D 3D

Nanoparticles Nanowire/nanotube Nanosheet Bulk material

Quantum dot Carbon nanotube, Si nanowire Graphene, MoS2 Any micron-size object

discovery of 2D materials like MoS2, MXene, silicene, germanene, etc. with interesting properties and applications. A 2D material consists of covalently bonded atoms arranged to form a sheetlike structure in XY direction, and the growth of such structure is not limited to nanometer scale; whereas the thickness of the sheet is confned to one or few atoms in Z-direction via weak van der Waals interaction [8–10]. The atomic arrangement shows in-plane strong bonding; whereas the interlayers are connected by the outplane weak bonding. Such structural arrangement of atoms in 2D materials makes them unique due to their unprecedented chemical and physical properties that are unparalleled compared to their bulk counterpart [8–10]. For example, graphene consists of the arrangement of carbon atoms bonded covalently in-plane, forming a sheetlike structure. The layer is limited mostly to one atom and can be extended to a few atoms. Graphene shows better functional properties, such as higher electronic mobility and mechanical strength, compared to the bulk counterpart, graphite [11]. The 2D materials are unique compared to their bulk counterpart for the following reasons: 1. High surface-area-to-volume ratio: The surface area, which is defned as the open area for chemical interactions, of a 2D material is higher compared to its bulk counterpart, leading to higher surface-area-to-volume ratio. High surface area 2D materials have more atoms of unsaturated bonds, which is benefcial in terms of faster chemical reaction, higher catalytic effect, and better sensitivity to external species [12]. 2. Exclusion of weak van der Waals interaction: A normal bulk material having layered crystal structure shows arrangement of atoms with in-plane covalently bonding, forming sheetlike structure, where these sheets (or layers) are held together by weak van der Waals interaction [8–10]. The application of external forces can easily outplay the weak van der Waals interaction, leading to breaking of these materials. In case of a monolayer or few-layer 2D materials, the in-plane atoms are connected by strong covalent bonds and seem to have better mechanical strength due to the removal of weak van der Waals interaction or the negligible presence of weak van der Waals interaction. For example, graphene, which is a single or few-layered 2D material exfoliated from graphite, shows higher mechanical strength, i.e., tensile strength 1,000 times greater than graphite. It can be noted here that the graphene even appears 100 times stronger than the steel [13].

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3. Localization of electron, known as quantum confnement: As mentioned earlier, the functional properties of a material are the function of the geometrical dimensions. Restriction in size/dimension of a material to nanometer scale leads to electron confnement in the direction where the dimension has been reduced. Such electron confnement can modify the band structure of a material, thereby changing the functional properties. Band structure of a material has great impact on the functional properties of a material, mainly electrical and optical properties. It has been seen that graphene shows extremely high electrical conductivity compared to graphite due to modifed band structure, where the electrons are confned in the perpendicular direction of graphene surface [14]. The fuorescence and sensing properties of MoS2 monolayer has been improved compared to bulk MoS2[15]. With the change in properties due to limiting in dimension, 2D materials are often well accepted for industrial application, where their counterpart bulk materials fail to perform. They fnd a wide range of applications, such as fexible transistor, sensor, photodetector, and energy storage (electrode for batteries and supercapacitors) [8–10]. The next section shows some interesting applications, where 2D materials outperform their bulk counterpart. 1. Flexible transistor: 2D materials having ultra-low thickness are considered as potential class of materials for fexible electronics. Their versatile electronic properties and high breaking strain make them suitable for fexible electronics applications. The 2D metal chalcogenides, such as MoS2 and WSe2, show semiconducting properties, which make them suitable as channel materials for fexible electronics. Graphene showing high electrical conductivity is suitable for electrode materials. The 2D materials that have a high bandgap, such as 2D hexagonal boron nitride (h-BN), are suitable for dielectric gate applications [16]. 2. Pressure or strain sensor: 2D materials are subjected to change in bandgap under applied pressure or strain. The existing distortions in 2D material may contribute to the additional scatterings, leading to lower electrons’ mobility. The combined factors modulate the resistivity of the 2D material, known as the piezoresistive effect. The pressure or strain sensor is based on the principle of piezoresistive effect [17]. 3. Photodetectors: For photodetectors, materials having suitable bandgap energy close to optical or near infrared region and excellent transport properties are highly essential [18]. In 2D materials, bandgap energy can be easily tuned while controlling the growth of the number of layers. Various metal chalcogenides, such as MoS2, MoSe2, and WSe2, show bandgap energy close to optical or near infrared region with excellent transport properties, which is required for ideal photodetector materials [19]. The device based on monolayer MoS2 and combined graphene/MoS2 heterostructure has shown 103–108 A/W sensitivity in 400–680 nm spectral range [19]. 4. Electrode materials batteries and supercapacitors electrodes: Materials having high electronic conductivity and high specifc capacity are required

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for both batteries and supercapacitors [20, 21]. 2D materials are known to have high electronic conductivity along with high surface-to-volume ratio, which helps to store more ions, leading to high specifc capacity. In addition, they will provide a faster charge transport along their surface, which will enhance the rate performance. MoS2 and graphene have received huge attention as electrode materials for batteries and supercapacitors [20, 21]. As discussed, the 2D materials fnd wide range of applications due to their enriched functional properties, which fully depend on the synthesis routes. The synthesis remains the key to develop the high-quality 2D materials in large scale for industrial applications.

2.2

SYNTHESIS OF 2D MATERIALS

Synthesis of materials is the key to design advanced materials of different shapes and sizes to achieve improved functional properties. In literature, various approaches were adopted to prepare different classes of materials from bulk to nanometer scale, limiting the geometrical dimensions; it may be in one dimension or in all three dimensions [8–10]. Basically, 2D materials are synthesized either from its bulk counterpart by thinning or nucleating to one or few atomic layers. To create 2D materials, mainly layered materials are preferred due to the presence of weak van der Waals interaction between two layers. Hence, such materials can easily be exfoliated or thinned to one or few-layered 2D materials. The synthesis routes used for the preparation of 2D materials are broadly classifed into two different categories, such as top-down (which explains synthesis of 2D materials by thinning or exfoliating the bulk materials) and bottom-up (which explains synthesis of 2D materials by the nucleating or assembling of atoms) [22]. These broad categories of synthesis routes include various specifc approaches adopted for the synthesis of 2D materials, and their advantages and disadvantages are explained.

2.2.1 TOP-DOWN APPROACH 2.2.1.1 Mechanical Exfoliation Mechanical exfoliation—the term refers to exfoliation or disintegrating the smaller unit from the bulk by means of mechanical force applied to the parent samples. It can also be termed as thinning of the sample to small units. Basically, this approach has been used for the preparation of 2D materials in a laboratory. Mainly layered materials having weak interlayer van der Waals interaction are chosen for this synthesis routes [23]. A Scotch tape is used to peel the material from the bulk sample by means of mechanical force. The sticky tape is pressed onto the surface of the bulk material, which peels the smaller units due to the adhesive force. This method was frst used to prepare the 2D graphene from graphite. Novoselov et al. reported the synthesis of single- or few-layered graphene from graphite using the Scotch tape approach [23]. Jayasena et al. reported the use of diamond wedge for mechanical exfoliation of graphene from graphite, known as highly oriented pyrolytic graphite (HOPG) [24]. They used ultra-sonication in addition to mechanical exfoliation to get high-quality

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graphene, similar to the approach adopted by Geim et al. [6]. The control in ultrasonication oscillations led to the consistency in the properties of graphene. The synthesis of defect-free graphene has been reported by Keith et al., where they replaced the ultra-sonication by the shear exfoliation in stabilizing N-methyl-2-pyrrolidone (NMP) [25]. In this process of exfoliation, the graphite is immersed in NMP and mixed at shear rate higher than 10,000 s−1. However, mechanical exfoliation of graphene has various shortcomings, such as uncontrolled synthesis of graphene with formation of many layers and defects in graphene created during exfoliation. For these reasons, mechanical exfoliation remains popular for lab-based studies; however, it is not scalable for integration into new technologies. 2.2.1.2 Exfoliation via Ion Intercalation Unlike mechanical exfoliation, a better route, known as ion intercalation exfoliation, was extensively used for the synthesis of 2D materials like graphene and MoS2 [26]. The exfoliation through ion intercalation has been adopted as a potential route to synthesize 2D materials, where a smaller size alkali ion, like lithium, is inserted into the interlayer gap of 2D material, which helps in widening the gap and weakening the van der Waals interaction [26]. The ion intercalated materials are subjected to reaction with various aqueous/nonaqueous solutions, starting from water to organic solvent materials, in order to release the gas, which helps to achieve 2D materials suspended in solution. In a few cases, heating of the solution is carried out to achieve better results. However, the prolonged reaction time (two to three days) and uncontrolled ion interaction into materials are the two big challenges in the synthesis of 2D material though this route. Single- or few-layers MoS2 have been synthesized through this route [26].

2.2.2 BOTTOM-UP 2.2.2.1 Soft Chemical Synthesis Route Solution-based chemical route is widely used for synthesis of nanomaterials. The atomic scale mixing of the precursor materials can provide high-quality nanomaterials. Such unique synthesis route has been extended to synthesize the 2D materials. Mainly, graphene and various transition metal dichalcogenides (TMDCs) are synthesized using solution-based chemical routes [27]. These include interface mediated growth, solvothermal, hydrothermal, and microwave-assisted chemical routes. Solution-based chemical routes are benefcial in terms of large-scale synthesis of 2D materials, low-cost, and their versatility. However, residual solvent remains a major issue [27]. 2.2.2.2 Chemical Vapor Deposition (CVD) Route The CVD technique is an effective route to synthesize the nanomaterials of high quality and controlled morphology. Through controllable process parameters offered by CVD techniques, 2D materials of high quality and improved functional properties can be achieved. The 2D materials like graphene and various TMDCs have been synthesized through this approach [28]. Not only pure 2D materials, their composites

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can be synthesized through CVD by using various precursor materials, even at large scale. CVD involves growth of 2D materials on the surface of a heated substrate placed within a furnace. The temperature of the substrate is fxed based on the type of materials to be grown. The gaseous precursors along with the career gases are passed through the heated furnace, where the precursors react with the heated substrate to form the flm [28]. The synthesis process has successfully been used to synthesize graphene, TMDCs, etc. To achieve the high-quality 2D materials of required thickness and composition, the process parameters can be easily tuned, which is a great advantage in CVD. Based on the previously discussed synthesis routes, different 2D materials were synthesized to integrate them into devices. However, the discussion on the synthesis of few 2D materials likes TMDCs, MXenes, and silicene are in the scope of this chapter.

2.3

SYNTHESIS OF TRANSITION METAL DICHALCOGENIDES (TMDC)

Transition metal dichalcogenides (TMDC) are the potential class of materials due to their wide variety of applications, such as sensor and optical materials [29–31]. The properties of these TMDCs can be improved through designing their nanostructures, while the 2D TMDCs have shown excellent properties [29–31]. Synthesis of 2D TMDCs remains crucial to achieve high-quality materials with improved functional properties. Various synthesis routes, such as mechanochemcial exfoliation [32], solution-based exfoliation [33], ion intercalation exfoliation [34], and chemical vapor deposition (CVD) [35], were extensively used for the synthesis of 2D TMDCs. The mechanical exfoliation is considered as one of the best approaches to synthesize 2D TMDCs. The 2D MoS2 synthesized through this approach have shown excellent photoluminescence properties to date [32]. In addition, solvent-based exfoliation has been used to synthesize the 2D TMDCs. Coleman et al. [33] reported the preparation of few-layer MoS2 and WS2 using a surfactant-free liquid-exfoliation route. Low boiling point solvent mixture was also used for the exfoliation process for the synthesis of 2D TMDCs. Another effective route for the synthesis of 2D TMDCs is the ion intercalation exfoliation. Zeng et al. [34] reported the synthesis of single-layer MoS2, WS2, TiS2, and ZrS2 though lithium ion interaction exfoliation process. However, the process involves prolonged reaction time with environmental impact. In addition to these synthesis routes, chemical vapor deposition (CVD) [35] and pulse laser deposition (PVD) [36] were extensively used for the synthesis of single or few-layer 2D. The detailed synthesis of 2D TMDCs using various chemical and physical routes has been explained elsewhere [22].

2.4 SYNTHESIS OF MXENES Recently, MXenes have been considered as potential class of materials, which are basically two-dimensional inorganic compounds consisting of a-few-atoms-thick layers of transition metal carbides, nitrides, or carbonitrides [37, 38]. MXenes are prepared from the MAX having hexagonal crystal structure with P63/mmc space

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group, where the M is transition metal, A is mainly Al (or elements from group 13 or 14 of the periodic table) and X is C, N, or CN [37, 38]. MXenes are typically prepared from MAX by selective etching of A layer using an etchant like HF [37]. During this process, the MAX powder is mixed with HF solution and stirred for a few hours at room temperature to expedite the process of etching, where selective etching of A layers takes place. While converting MAX to MXene, the metallic bonding is replaced by the weak interlayer bonding. Non-MAX phase, such as Mo2Ga2C, is also used for the synthesis of Mo2C MXene, where the Ga layer is being etched [39]. Apart from selective etching, MXenes are also prepared by intercalating the guest molecules or ions into the layered MAX phases that have weak interlayer bonding. The insertion of guest molecules or ions causes lattice expansion in MAX and, hence, increases the c-lattice parameter, further weakening the interlayer bonding and fnally resulting in exfoliation of MXene layers [38]. For instance, hydrazine (N2H4) has been intercalated into Ti3C2(OH)2, forming Ti3C3MXene [38].

2.5

SYNTHESIS OF SILICENE

The silicene, which is the graphene version of silicon, was frst investigated theoretically by Shiraishi and Takeda in 1994 [40, 41]. The silicene is different from graphene in crystal structure point of view, where the two sublattice A and B does not lie in the same plane and is shifted in the perpendicular direction to the atomic plane [40, 41]. In 2007, Guzman-Verri and Lew Yan Voon named such structure as silicene [40]. The synthesis of silicene still remains a challenge, unlike graphene, due to various diffculties, such as lack of availability of layered silicon and also the highly insulating nature of silicon oxide; hence, mechanical and chemical exfoliation techniques cannot be employed. Mainly, CVD technique has been employed as potential synthesis route to prepare silicene [40]. Si nanomaterials with spherical shape, consisting of freestanding 2D Si nanosheets, were prepared by CVD technique. The thickness of these Si nanosheets was maintained below 2 nm [40]. Solution-based chemical routes were also adopted to prepare 2D silicene [41]. Nakano et al. [42] reported the synthesis of silicene through the use of magnesium doped hexagonal CaSi2. The exfoliation of CaSi0.85Mg0.15 in the presence propylamine hydrochloride (PA·HCl) solution resulted in silicene with composition of Si: Mg: O = 7.0: 1.3: 7.5 as confrmed using XPS. Okamoto et al. [43] reported the synthesis of oxygen-free Si nanosheets of thickness below 2 nm via exfoliation of layered Si6H6, while reacting with n-decylamine. The obtained Si nanosheet exhibited excellent photoluminescence properties. The synthesis of silicene (or silicon nanosheets) via solution-based routes appears more diffcult compared to other 2D materials.

2.6 SUMMARY In summary, 2D materials are considered as potential class of materials due to their rich functional properties achieved through the presence of high surface-to-volume ratios and excellent electronic transport. They fnd wide applications as a catalyst, fexible transistor, pressure sensor, and electrode material for supercapacitors and batteries. The localization of electrons in the direction perpendicular to the surface

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further adds to their excellent optical properties due to the modulation of bandgap. Hence, it is highly essential to opt for advanced synthesis routes to prepare highquality 2D materials in large scale. Two broad synthesis approaches, such as topdown and bottom-up, have been used for the synthesis of 2D materials. The top-down approach includes micromechanical, ultra-sonication, and ion intercalation exfoliations. The bulk materials having layered structures are preferred for the synthesis of their 2D materials through top-down approach. In the case of bottom-up approach, the synthesis of 2D materials occurs by the nucleation or the assembling of atoms. The bottom-up approach includes chemical vapor deposition (CVD), soft chemical routes, such as microwave-assisted chemical, solvothermal, and hydrothermal routes. In future, efforts should be made to develop advanced synthesis routes to prepare high-quality and defect-free 2D materials in large scale for industrial applications.

ACKNOWLEDGMENT The authors would like to acknowledge the fnancial support from the Department of Science and Technology through Technical Research Centre (TRC project: AI/1/65/ARCI/ 2014) and DST project: DST/TMD/MES/2K17/46, Government of India, for the completion of the work. The authors would like to thank Dr. Tata Narasinga Rao, director (additional charge), ARCI, for his support to complete the work.

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Electrical Properties of Graphene Comprehensive Study Sourav Sarkar, Samik Saha, and Sachindranath Das

CONTENTS 3.1 3.2 3.3

Introduction .....................................................................................................17 Origin of the Exceptional Electrical Property of Graphene............................19 Electrical Properties of Single-Layer Graphene on a Substrate ..................... 20 3.3.1 Impact of Temperature.........................................................................21 3.3.2 Effect of Electric Field.........................................................................21 3.3.3 Effect of Pressure................................................................................ 22 3.3.4 Effect of Doping and Absorption of Nanoparticle ............................. 23 3.3.5 Effect of Defect................................................................................... 24 3.4 Electrical Properties of Bilayer and Multilayer Graphene ............................. 25 3.4.1 Bilayer Graphene ................................................................................ 25 3.4.1.1 Impact of Sample Length..................................................... 25 3.4.1.2 Impact Due to the Angle of Twist........................................ 26 3.4.1.3 Effect of Impurity, Chemical Potential, Temperature, and Frequency...................................................................... 27 3.4.2 Multilayer Graphene ........................................................................... 30 3.4.2.1 Effect of Temperature and Twisting Angle of Multilayer Graphene ............................................................ 30 3.4.2.2 Impact of Chemical Potential, Frequency, Temperature, and Impurity Level............................................................... 30 3.4.2.3 Variation in the Mobility with Layers Number ................... 34 3.5 Conclusion ...................................................................................................... 36 References................................................................................................................ 36

3.1

INTRODUCTION

Graphene is a common name in the research feld due to its exceptional and interesting mechanical, thermal, optical, and electrical properties. It is a stable form of carbon atoms, wherein atoms are arranged in a two-dimensional hexagonal lattice in a picturesque manner. Single-layer graphene was theoretically defned by P. R. Wallace in 1947 [1]. It has been studied since its inception; however, it was frst unambiguously produced and identifed in 2004 [2] by Andre Geim and Konstantin Novrselove. They received the Nobel Prize in physics in 2010 for their groundbreaking experiments. DOI: 10.1201/9781003247890-3

17

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FIGURE 3.1 Different forms of carbon.

FIGURE 3.2 Use of graphene in different felds.

Graphene belongs to the family of different graphitic materials like carbon nanotube (CNT) and fullerene, as shown in Figure 3.1. Graphene has a large specifc surface area, high thermal conductivity, high electrical conductivity, and good fexibility. Graphene frst attracted the curiosity of physicists owing to its particular electronic behavior under magnetic felds and at low temperatures. Graphene is stronger than steel but is a much lighter material. Because of its attractive properties, it has many potential uses in different felds. In the feld of energy research, graphene has been used as an electrode material for different electrochemical energy storage devices. Graphene can be used to make supercapacitors. Graphene has also found applications

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19

in the feld of biomedical and environmental research. Figure 3.2 shows the different felds where graphene can fnd potential applications. In ideal graphene, there is no gap between the valence band and conduction band [3]. Hence, unmodifed graphene is not useful in the feld of semiconduction. To modify the band structure of graphene, nitrogen (N), boron (B), etc. have been used as dopant. It is an almost transparent material with high electrical conductivity. It is one of the promising materials for optoelectronic and nanophotonic devices. Graphene can also be used to fabricate sensors, like biosensors, pressure sensors, and magnetic sensors. Graphene can be categorized as very-few-layer graphene (vFLG), having one to three layers of carbon; few-layer graphene (FLG), with two to fve layers of carbon; and multilayer graphene (MLG), with more than fve layers of carbon. In this chapter, we are going to focus on graphene’s electrical properties and the impact of different factors on it.

3.2

ORIGIN OF THE EXCEPTIONAL ELECTRICAL PROPERTY OF GRAPHENE

Experimental studies on graphene have revealed exotic transport properties. As we have mentioned earlier, graphene is a two-dimensional carbon allotrope. Carbon atoms form a honeycomb-like arrangement. The hexagonal lattice of graphene can also be regarded as two interleaving triangular lattices. Each carbon atom is situated at a distance 1.42 Å from its three nearest neighbor carbon atoms. The carbon atom has four outer cell electrons. Three out of these four electrons occupy sp2 hybrid orbital in graphene. Three electrons reside in a 2D plane, and the carbon atom wants to share these three electrons with their neighboring three carbon atoms and thus form σ bonds, as shown in Figure 3.3 (a). These bonds make graphene a strong and fexible material. The fourth electron makes a π-bond, which is oriented in the out-of-plane direction. Each carbon atom has one of this out-ofplane bond. Geometrically, the π bond is like a dumbbell-shaped symmetric lobe having the carbon nucleus at the center and oriented along the z-axis. The fourth

FIGURE 3.3 (a) Sublattice structure of graphene. (b) Band structure of graphene in the frst Brillouin zone. Source: Reproduced from ref. [5], Cooper et al. is an open access distributed under the creative attribution license, copyright 2012.

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electron can reside in the lobe oriented along either the positive z-axis or the negative z-axis. This electron is free to move along the plane and is responsible for the exotic electronic properties. In 1947, Wallace theoretically determined the band structure of single-layer graphene [4]. In ideal graphene, there is no gap between the valence band and conduction band as these bands touch at the Dirac points. The Dirac points are located at the edges of the Brillouin zone. In Figure 3.3 (b), the band structure of graphene in the frst Brillouin zone is shown. There are six Dirac points, and these points can be divided into two sets, each having three points. The two sets are completely different from each other and can be pointed out by K and K /. The two sets of Dirac points give graphene a valley degeneracy of g = 2 [5]. Transport in graphene displays a novel chirality. Each graphene sublattice forms a branch of dispersion. These dispersion branches make very weak interactions with one another. The chiral effect determines the presence of a pseudospin quantum number for the charge carrier. This quantum number has similarity with spin, but it is not dependent on the real spin. Using pseudospin, the contributions from each of the sublattices can be recognized [6]. According to Dirac Hamiltonian, the charge carriers cannot be constricted using electrostatic potentials. In semiconducting material, if an electron encounters an electrostatic potential barrier with a barrier height higher than electron’s kinetic energy, then the electron’s wave function would decay exponentially within the potential barrier and may reach the other side depending on the height and width of the potential barrier. This can be shown for wider and taller barriers, the tunneling probability for the electron reduces. However, if the particle follows the Dirac points, then the transmission probability will increase with increasing barrier height. In the case of graphene, this situation would be different from quantum tunneling. When electrons run upon a barrier, it would change into a hole that moves through the barrier [5]. When the hole reaches another side of the barrier, then it would change into an electron. This phenomenon is defned as Klein tunneling. In this case, the mode matching between the electron outside the barrier and hole within the barrier increases with the height of the barrier. It is exactly the opposite of quantum tunneling probability. In the case of perfect mode matching, perfect transmission can be seen. In graphene, the transmission probability can be controlled with the angle of the incident to the barrier [7]. In brief, ideal graphene has zero bandgap, which can be tuned by doping. It shows Klein tunneling and follows linear dispersion relation. These facts are behind the attractive physical and electrical properties of graphene.

3.3 ELECTRICAL PROPERTIES OF SINGLELAYER GRAPHENE ON A SUBSTRATE The suspended graphene offers higher charge carrier mobility compared to graphene deposited on a substrate. However, from the device point of view, it is essential to study the electrical properties of graphene deposited on a substrate. Different substrates like silicon dioxide (SiO2), silicon carbide (SiC), hexagonal boron nitride (h-BN), etc. have been used as a substrate to hold graphene as a fat layer limiting the self-corrugation. The electrical properties thus measured depend on several parameters and are discussed in the next section.

Electrical Properties of Graphene

3.3.1

21

IMPACT OF TEMPERATURE

Schiefele et al. theoretically studied the effect of temperature on the electrical conductivity of graphene deposited on hexagonal boron nitride (h-BN) and compared with the same deposited on SiO2 and SiC. Different mechanisms infuence the conductivity and carrier mobility of deposited layer of graphene. The nature of substrate determines such mechanisms. It is found that optical surface phonon plays a major role in determining the charge carrier mobility of graphene deposited on SiO2 substrate. In such case, the temperature dependence of the conductivity consists of two contributions: First one is the scattering by acoustic phonons in graphene, and the second one is the scattering by surface phonons of the substrate. In the case of h-BN, remote-phonon scattering plays a major role at room temperature. Resistivity of graphene sheet on h-BN increases with the increase in temperature because of the dependence of scattering of electrons with the surface phonons. With the increase of temperature, the surface phonons’ population increases, which increases the rate of electron scattering. Figure 3.4 shows the variation of resistivity of graphene layer on different substrates [8].

3.3.2

EFFECT OF ELECTRIC FIELD

To study the electrical properties of graphene in the presence of an external electrostatic feld, generally, a four-probe method based on feld-effect transistor (FET) principle is used. The electrical properties can be tuned using an electrostatic feld. FET consists of a source terminal and a gate terminal, as shown in Figure 3.5 (a). A conducting channel exists between these two terminals. Usually, the channel contains a thin layer of graphene. There is a third electrode known as gate, which is kept separated from the channel by a dielectric material. The gate electrode and the channel form a geometry resembling a parallel plate capacitor. When a gate voltage (Vg) is applied to the gate terminal, the capacitor, like geometry, induces charge into the channel. The presence of this electric feld changes the position of the Fermi level and the carrier density, and thus, the electrical properties of the channel could be changed. Reportedly, a change of carrier density (two-dimensional) of the order

FIGURE 3.4 Variation in graphene resistivity with temperature (for three different substrates). Source: Reproduced from ref.[8] with permission from American physical society, copyright 2012

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FIGURE 3.5 (a) Geometry of FET. (b) Four-probe method. (c) Variation of carrier density vs. gate voltage. Source: Reproduced from ref.[10] with the permission American Institute of Physics, 2010

of 1012–1013/ cm2 can be incorporated in the channel using a dielectric separator of SiO2 [9]. This process of modulation of the electric feld is known as the electrostatic doping. As the process is physical and involves no direct charge transfer, it is highly reversible and induces no defect in the channel lattice. Dorgan et al. examined the mobility and saturation velocity of charge carriers in graphene on SiO2 substrate [10]. For this purpose, a four-probe method was used on a modifed FET-like structure, where s highly doped Si substrate acted as a back gate. The schematic of the setup is shown in Figure 3.5 (b). From Figure 3.5 (c), it can be seen that the carrier density can effciently be controlled by the application of the gate voltage.

3.3.3

EFFECT OF PRESSURE

Graphene can sustain ultrahigh pressure (~ 130 GPa) because of the strong in-plane bondings [11]. However, application of high pressure can modify the local electrical properties of graphene layer deposited on a substrate. Ares et al. studied the effect of pressure on monolayer graphene deposited on SiO2 substrate [12]. Ultrahigh pressure was obtained using an atomic force microscopy (AFM) having a diamond tip cantilever. This simple setup can be used to reach a pressure level of ~ 40 GPa. Initially, graphene fakes were deposited on SiO2/Si substrate. Optical microscopy along with the use of Raman spectroscopy enabled the authors to locate the different areas of the substrate where a single layer of graphene has been deposited. Using the diamond tip of the AFM cantilever, ultrahigh pressure was applied to these different zones having a single layer of graphene. Due to the application of pressure, these regions were modifed. An AFM image revealed a formation of valleys in these regions. The depths of the valleys were different for different applied pressures. It was seen that for pressure less than 13 GPa, no valley could be formed. For pressure within 16 to 25 GPa, a valley with a depth of ~ 1 nm was observed. Upon applying even more pressure (29–37 GPa), the depth of the valley increased to 1.3 nm. However, application of pressure greater than 40 GPa broke the graphene sheet. The modifed regions were

Electrical Properties of Graphene

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later analyzed using Kelvin probe force microscopy (KPFM). KPFM is a noncontact type variant of AFM, which can be used to measure the work function of the surface at an atomic scale. It was seen that as the pressure was increased, the contact potential difference reduced. This signifes that the Fermi level moves down as the pressure is increased. This is equivalent to hole doping, and thus the whole process can be called pressure-dependent doping in graphene. It was asserted that the application of pressure resulted in the formation of strong covalent bonds between graphene and the SiO2 substrate. These bonds shifted the position of Fermi energy, and thus electrical properties of graphene can be tuned. However, when the same experiment was repeated on the few-layer graphene, no such shift in Fermi energy was observed. This is mainly because of the lack of bonds between the different carbon layers.

3.3.4

EFFECT OF DOPING AND ABSORPTION OF NANOPARTICLE

We know that the mobility of ideal graphene is much higher than that of semiconducting material. Consequently, graphene is considered as a promising material for applications in electronics. However, as already mentioned, pristine graphene has no bandgap, and most electronic applications are hindered for this reason [3]. Doping of atoms is an effcient way to control the graphene bandgap. There are several methods, using which, we can tune the same. Heteroatom doping: In this method, boron (B) and nitrogen (N) atoms are used [13–18]. The size of B atom and N atom are similar to the size of the carbon atom, and these atoms have hole-acceptor and electron-donor characters. These properties make these atoms suitable as doping elements. After doping with B and N atoms, graphene behaves like p- and n-type semiconducting material, respectively. First principle density functional theorem shows that the electronic bands of these doped graphene are almost similar to pristine graphene within 1 eV of the Fermi energy and displays linear dispersion relation. It is relatively diffcult to introduce foreign atoms in the perfect 2D structure of graphene; however, there are a few methods of B- and N-doping in graphene, like the arc discharge method, chemical vapor deposition method (CVD), etc. The resistance of the doped graphene decreases with the increase in temperature, indicating its semiconducting property and the carrier mobility of doped graphene is lower than that of the pristine graphene [3]. Temperature-dependent resistivity measurement of N-doped graphene shows that the resistance reduces by a factor of 80 when the temperature is increased from liquid N2 temperature to 300 K. Chemical modifcation: In this case, graphene electrical properties are modifed by NO2 and NH3 treatment [3]. Using NO2 and NH3, bandgap can be created in graphene. After doping, graphene behaves like a p-type and n-type semiconductor, respectively. Water vapor absorption also signifcantly infuences the electrical properties of graphene. Graphene flm, when exposed to humidity, develops a maximum bandgap of 0.206 eV. Ability to induce a different bandgap depending on different chemicals makes graphene a strong contender for sensor application. Impact of absorption of nanoparticle: Nanoparticle (both organic and inorganic) absorption on single-layer graphene can tune the electrical properties. Tanaka et al. synthesized a small stripe of single-layer graphene, also known as graphene nano

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ribbon (GNR), from double-walled carbon nanotubes (DWNTs). As the name suggests, DWNTs have two concentric cylinders, which can be made by rolling two stacked graphene layers in the form of a cylinder. While synthesizing GNR, the authors used chemical treatment followed by sonication to unzip the two cylinders. Thus, a stripe of bilayer graphene was produced. Further sonication separates the bilayer graphene into two separate single-layer graphene. They further deposited nanoparticles of an organic molecule (C15-NDI) on the single layer of graphene. The nanoparticles were attached to the graphene layer via π-π stacking. As the p-electron is the major contributor to the electronic conductivity of graphene, hence by varying the number of nanoparticles, the electrical properties can be tuned [19]. If the absorbed molecular number on GNR is higher than that of conduction electrons, the electrons would be trapped by the nanoparticles. The conduction path of the electron will become narrower as the size of the absorbed nanoparticle increases. The I-V characteristic curve of GNR reveals its semimetallic nature, whereas after the adsorption of nanoparticles, it behaves like a p-type semiconductor. The bandgap of GNR is similar to the semiconductor [20].

3.3.5

EFFECT OF DEFECT

Ideal graphene has one atomic layer where carbon atoms are situated at the vertices of two-dimensional hexagonal lattice structures. However, it is not possible to experimentally synthesize the symmetric hexagonal lattice of carbon atoms. Experimentally obtained graphene contains different types of defects. These defects break the structural symmetry. The defects in graphene can be classifed as intrinsic and extrinsic. When the crystal structure is perturbed without the presence of foreign atoms, the defect is known as intrinsic defect. On the other hand, when external foreign atoms induce defects in the lattice structure, then the defect is known as extrinsic defect. The defects are not always stationary and can migrate over the crystals. This propagation of defect can infuence the different properties of the crystal [21]. Stone-Wales (SW) defect is a kind of point defect in the graphene structure, which is intrinsic in nature. Sometimes within the graphene lattice, there could be a few areas that can contain nonhexagonal rings. This is known as SW defect. Single or multiple vacancies is also an intrinsic-type defect. If in the lattice of graphene there are missing atoms in certain areas, then it is known as a vacancy defect. Single vacancy (SV) is formed when only one carbon atom is missing from a lattice. Multiple vacancies arise with two or more SV coalescence. Theoretically the formation energy of DV is of the same order as SV, and it is around 8 eV. There are different types of extrinsic defects. When a foreign atom is incorporated in the graphene lattice, it is known as the defect due to foreign adatoms. The foreign atom may form covalent bonding with the nearest carbon atom if the interaction between that atom and a carbon atom is strong. This is known as chemisorption. However, if the interaction is weak, then only physisorption of the atom occurs through van der Waals interaction. How the property of the graphene will be modifed depends entirely on the type of bond the atom forms with the carbon atoms. Foreign atoms can also be incorporated into the lattice, replacing one or more carbon atoms from the lattice. Boron or nitrogen is usually used in this type of substitution.

Electrical Properties of Graphene

25

Topological defect arises when the graphene deviates from a planar structure and forms hills or valleys at some places, imposing curvature in the lattice structure. Due to curvature, the bond angle is changed, which changes the properties of the graphene. The electronic properties of graphene are strongly infuenced by the presence of defects. In theory, to obtain the quantum mechanical solution of the electronic wave function, we have to use Dirac equation in place of Schrodinger equation. However, in the presence of a defect, the Dirac equation has to be modifed, which changes the electronic wave function of the electrons in the lattice. All these defects act as scattering centers for the electron wave and changes the trajectories of the electrons. Thus, decrease in conductivity is observed with the increasing defect density. Point defects create local states near the Fermi energy level in the sp2 orbital. SW defect can create local bandgap up to 0.3 eV in width. However, the effect on conductance due to more complicated defects (which comprises of nonhexagonal rings) has not been studied till now, and more research is necessary to gain more insight into this feld.

3.4 3.4.1

ELECTRICAL PROPERTIES OF BILAYER AND MULTILAYER GRAPHENE BILAYER GRAPHENE

Bilayer graphene (BLG) is a pile of two monolayer graphene, where two layers are stacked vertically. Fabrication of monolayer graphene is diffcult. On the other hand, bilayer graphene can be fabricated relatively easily, and thus it is used instead of monolayer graphene in different electronics devices. Electrical property of bilayer graphene varies with different parameters, like sample length, twisting angle between two layers, temperature, frequency, chemical potential, etc. 3.4.1.1 Impact of Sample Length The ensemble average conductance of the opened bilayer graphene depends on the length of the sample and d, depth from starting point. Some attractive properties originate from the edge state of graphene, like valley-dependent transport and electric feld tunable magnetism. It is diffcult to fabricate perfect edge; some disorder must be there in the edge of graphene. Removing the tight binding randomly between two layers of graphene creates structural edge disorder in the bilayers, till depth d from starting point. Transport property of low energy states of BLG has been discussed. The conductance of gapped bilayer graphene can be tuned. The ensemble average conductance of bilayer graphene can be altered with the length of the sample (L) for different values of edge disorder (d). The conductance of different types of edges, like zigzag edge, armchair, and other edges, depends on the length of the sample (L) for different values of edge disorder (d). Zigzag, armchair, and other edges can be created by cutting bilayer graphene along arbitrary crystallographic direction. In Figure 3.6, zigzag edge and armchair edge have been shown [22]. Carbon atom of zigzag edge is not chemically stable. It is prone to combine with other reactants because it has an unpaired electron. But the carbon atom of an

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FIGURE 3.6

Emerging 2D Materials and Applications

(a) Zigzag edge of graphene. (b) Armchair edge of graphene.

Source: Reproduced from ref. [22] with permission from American Chemical Society, copyright 2010

armchair edge is chemically stable owing to its triple covalent bond. Edge disorder has infuence over transport properties of graphene nanoribbon with different types of edge character like zigzag and armchair. Zigzag edge and armchair edge show different types of standing wave pattern, and it can be probed by using scanning tunneling spectroscopy [23]. For long-range impurities scattering zigzag graphene nanoribbon shows perfect conductance. In single-mode regime, we get different transport outlines for armchair graphene nanoribbon and zigzag graphene nanoribbon [24]. The transport properties in multimode regime have been discussed [24]. Practically, it has been found that disorder has less infuence over zigzag edge of BLG when it is close to an edge. This behavior is completely different when chiral edge is at zigzag boundary [25]. It is related to the effective boundary condition at the zigzag edge. In this case, zigzag edge does not mix valleys. That is why edge disorder has no infuence on conductance over zigzag edge. However, edge disorder has infuence on conductance over armchair edge because an armchair edge mixes valleys. In a many-mode regime, there is a small difference between the conductance of armchair and zigzag edge. Chemical disorder has randomly distributed energy +1 eV for chemical species of outermost binding of carbon atoms. The presence of chemical disorder or defect has infuence on the variation in conductance with length. The conductance of bilayer graphene decreases with the sample length exponentially, and it does not depend on the width of the graphene strip. Nevertheless, there is some exception; if the strip’s length is less than 5 nm, then the bulk contribution to conductance would be signifcant [26]. Moreover, the strong disorder has an impact on the variation in conductance of bilayer graphene with the length of the strip. In every case, the conductance varies similarly with sample length for zigzag edge, armchair, and other edges. 3.4.1.2 Impact Due to the Angle of Twist Bilayer graphene is a material where two monolayers of graphene are stacking vertically [27]. However, it is quite diffcult to fabricate perfect stacking of two monolayers. As a result, a small twist is observed between two layers. Consequently, electrical property of bilayer graphene varies with twisting angle between two layers. Several methods are used to fabricate twisted bilayer graphene. Each method has its advantages and disadvantages. But the stacking of two layer graphene prepared by chemical vapor deposition is basically followed [28]. Here, vertical stacking of two graphene

Electrical Properties of Graphene

FIGURE 3.7 singularity.

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Schematic band structure and the equivalent density of state with van Hove

Source: Reproduced from ref.[31] with the permission from Springer Nature, copyright 2016

layers was used [27]. As a result, symmetry of bilayer graphene was broken, and this broken symmetry created inequivalent charge and electrostatic potential between two layers. But twisted bilayer graphene is not stacked bilayer graphene, wherein one layer rotates with respect to other layer. In Figure 3.7, schematic band structure and the equivalent density of state with van Hove singularity is shown. Evidently, the Dirac band dispersion changes dramatically with the twisting angle when the twisting angle between two layers is less than 5°. The Dirac cone of each layer of bilayer graphene intersects and creates a saddle point in the shared space of twisted bilayer graphene. In consequence, a van Hove singularity is created in the density of state. A small twisting angle between two layers creates a pattern on intercalated atoms and modifes its electronic properties. A twisted bilayer graphene with small twisting angle shows high mobility. Yu et al. discussed the interlayer conductance varies with twisting angle. Twisted bilayer graphene displays a high interlayer conductance when the twisting angle is small [29]. This means that high interlayer conductance can be observed at zero twisting angle. As a result, the interlayer conductance of 0° twisting angle of bilayer graphene is higher than the 30° twisting angle of twisted bilayer graphene. This indicates that the decoupling and coupling transition creates a twisting angle depended interlayer contact conductance in twisted bilayer graphene. The measurement of electrical transport for different twisted bilayer graphene devices with different twisting angles of 0.75° to 2° in room temperature has been observed by Polshyn et al. [30]. 3.4.1.3 Effect of Impurity, Chemical Potential, Temperature, and Frequency Similar to single-layer graphene, conductivity of bilayer graphene also depends on the chemical potential, frequency, temperature, and impurity level [32]. Both the parallel and perpendicular conductivities to the plane in bilayer graphene have been investigated. The parallel and perpendicular conductivity shows different variation with chemical potential, frequency, temperature, and impurity level. At zero temperature and zero frequency, the in-plane dc conductivity of bilayer graphene varies with chemical potential. For the different levels of impurity, this variation

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FIGURE 3.8 Variation in dc conductivity of bilayer graphene with chemical potential at T = 0 K, ꙍ = 0.

Source: Reproduced from ref.[32] with the permission from American Physical Society, copyright 2008

would be different. Like for low impurity in bilayer graphene, the in-plane conductivity increases promptly with the increase of chemical potential. However, with the increase of impurity in bilayer graphene, this promptness will decrease gradually. In the case of perpendicular conductivity, the situation would be different. At zero temperature and zero frequency, the variation of perpendicular dc conductivity of bilayer graphene increases quickly for higher impurity. Even for higher impurity, an increase of perpendicular plane conductivity with chemical potential is greater than that of other lower impurity levels in bilayer graphene. For near zero impurity level in bilayer graphene, the perpendicular dc conductivity with chemical potential shows no variation. In Figure 3.8, the variation in dc conductivity of bilayer graphene with chemical potential has been shown at zero temperature and zero frequency. These fgures have been drawn in t-matrix and coherent-potential approximation (CPA). In CPA, curves show no peak, and curves are smoother than the curve in t-matrix. At zero temperature and zero chemical potential, the variation in in-plane conductivity of bilayer graphene with frequency gets slight variation, and then suddenly it reaches at pick point for particular value of frequency. However, this pick point decreases with increase of impurity. For further increase of frequency, the value of in-plane conductivity would decrease with frequency and then shows fat line. In case of perpendicular conductivity, it increases irregularly with frequency, and suddenly it

Electrical Properties of Graphene

29

FIGURE 3.9 (a) Variation in conductivity of bilayer graphene with frequency at µ  =  0 Ʌ, T = 0 K. (b) Variation in conductivity of bilayer graphene with frequency at T = 300 K, µ = 0 Ʌ. (c) Variation in conductivity of bilayer graphene with frequency at T = 0 K, µ = 0.025 Ʌ. (d) Variation in conductivity bilayer graphene with frequency at µ = 0.025 Ʌ and T = 300 K. Source: Reproduced from ref.[32] with the permission from American Physical Society, 2008

reaches at a maximum value. After reaching its maximum value, it reaches saturation point. In Figure 3.9 (a), variation in conductivity of bilayer graphene with frequency at µ = 0 Ʌ, T = 0 K is shown. At 300 K temperature and zero chemical potential, the variation of in-plane conductivity and perpendicular conductivity shows the same variations as variation at zero temperature. But for low impurity levels, the parallel conductivity shows a pick point. In Figure 3.9 (b), the variation in conductivity of bilayer graphene with frequency at T  =  300 K and µ  =  0 Ʌ is shown. At chemical potential µ  =  0.025 Ʌ and temperature T = 0 K, the in-plane conductivity of bilayer graphene dramatically varies with frequency. This variation decreases with increase of impurity levels in graphene. However, the value of in-plane conductivity reaches saturation level after a certain value of frequency. For a different impurity level, the variation would be different, but after a certain point, the in-plane conductivity reaches saturation point. In the case of perpendicular conductivity, the situation would be different. In Figure 3.9 (c) the variation in conductivity of bilayer graphene with frequency at µ = 0.025 Ʌ

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and temperature T = 0 K is shown. At µ = 0.025 Ʌ and temperature T = 0 K, perpendicular conductivity varies with frequency in an irregular manner. But after a certain value of frequency, the conductivity reaches saturation point. At T = 300 K and chemical potential µ = 0.025 Ʌ, both in-plane conductivity and perpendicular conductivity shows irregular variation with frequency for different impurity levels. In Figure 3.9 (d) the variation in conductivity of bilayer graphene with frequency at T  =  300 K and µ = 0.025 Ʌ is shown. At µ = 0 and ꙍ = 0, the variation in both direction varies with temperature for different levels of impurity. Only for a low level of impurity does the in-plane conductivity show variation. For a high value of impurity level, the in-plane dc conductivity maintains constant value with the variation of temperature. For higher impurity level, the perpendicular dc conductivity shows variation, and for lower impurity level, perpendicular conductivity maintains constant value with temperature. In Figure 3.10 (a) the variation in dc conductivity of bilayer graphene with temperature at µ = 0 and ꙍ = 0 is shown. At µ = 0.025 Ʌ, ꙍ = 0, both in-plane and perpendicular dc conductivity maintain nearly the same value with the increase of temperature. In Figure 3.10 (b) the variation in dc conductivity of bilayer graphene with temperature µ = 0.025 Ʌ, ꙍ = 0 is shown.

3.4.2

MULTILAYER GRAPHENE

Multilayer graphene is a pile of monolayer graphene layers where more than two layers are stacked vertically. Monolayer fabricates due to the same reason bilayer graphene is used instead of monolayer graphene. Electrical properties of multilayer graphene also vary with different external parameters, like monolayer and bilayer graphene. The electrical property of multilayer graphene depends on twisting angle, chemical potential, impurities, temperature, etc. 3.4.2.1

Effect of Temperature and Twisting Angle of Multilayer Graphene Some fascinating properties in twisted multilayer graphene (tMLG) originated from angular twists between the layers. Mogera et al. [33] described that the conductivity of the twisted multilayer graphene within the temperature range of 90 K to 273 K shows two different characteristics. The conductivity of each layer in MLG slowly increases with the increase of temperature and reaches a minimum value nearly at 180 K temperature and then linearly decreases up to the temperature 300 K. Two different natures (semiconducting and metallic) of MLG at different temperatures can be seen. In Figure 3.11, it shows the variation in conductivity of tMLG with temperature. 3.4.2.2 Impact of Chemical Potential, Frequency, Temperature, and Impurity Level The conductivity of multilayer graphene depends on the chemical potential, frequency, temperature, and different impurity levels in multilayer graphene [34]. Individually, each parameter has infuence over multilayer graphene, and collectively, all these parameters infuence the electrical properties of graphene. For multilayer graphene, conductivity is being considered in two different directions,

Electrical Properties of Graphene

FIGURE 3.10 (a) Variation in dc conductivity of bilayer with temperature at µ = 0 and ꙍ = 0. (b) Variation in dc conductivity of bilayer graphene with temperature, µ = 0.025 Ʌ, ꙍ = 0. Source: Reproduced from ref.[32] with the permission from American Physical Society, copyright 2008

31

32

Conductivity (e2/h)

Emerging 2D Materials and Applications

4.2

4.0

Semiconducting

3.8 0

FIGURE 3.11

Metallic

50

100 150 200 250 300 Temperature (K)

Variation in conductivity with temperature of twisted multilayer graphene.

Source: Reproduced from ref.[33] American Chemical Society, copyright 2017

like in-plane conductivity and perpendicular conductivity. Impacts of temperature, chemical potential, frequency, and impurity collectively on in-plane conductivity and the perpendicular conductivity are completely different. Because of that reason, in-plane conductivity and perpendicular conductivity of multilayer graphene show different variation [32]. In Figure 3.12 (a), it shows that at zero temperature and zero frequency, the in-plane conductivity of multilayer graphene varies with chemical potential. These fgures have been drawn in t-matrix and coherent-potential approximation (CPA). For the different levels of impurity, this variation would be different. For instance, for low-level impurity in multilayer graphene, the in-plane dc conductivity increases promptly with the increase of chemical potential. However, with the increase of impurity in multilayer graphene, this promptness will decrease gradually. In the case of perpendicular dc conductivity, the situation is quite similar to the variation for in-plane dc conductivity with chemical potential. At zero temperature and zero frequency, the variation in perpendicular dc conductivity of multilayer graphene increases quickly for low impurity level, and this promptness decreases with the increase of impurity. At zero temperature and zero chemical potential, the variation of in-plane conductivity of multilayer graphene with frequency gets variation. For higher-level impurity, the variation of in-plane conductivity is slightly greater than that of lower-level impurity. For lower impurity levels, the variation of in-plane conductivity is very little. After a certain value of frequency, the in-plane conductivity reaches saturation level for every level of impurity. The variation in perpendicular conductivity of multilayer graphene with frequency is quite different than that of in-plane conductivity of multilayer graphene. The perpendicular conductivity continuously decreases with an increase in frequency and reaches a minimum value, then again it increases with frequency. After a certain value of frequency, it reaches a saturation level. For a higher level of impurity, the minimum value of multilayer graphene is slightly higher than that of the lower level

Electrical Properties of Graphene

33

FIGURE 3.12 (a) Variation in dc conductivity of multilayer graphene with chemical potential, at T = 0 K, ꙍ = 0 with frequency. (b) Variation in conductivity with frequency of multilayer graphene, at T = 0 K, µ = 0Ʌ. (c) Variation in conductivity with frequency of multilayer graphene, at T = 300 K, µ = 0Ʌ. (d) Variation in conductivity with frequency of multilayer graphene, at T = 300 K, µ = 0.025Ʌ. Source: Reproduced from ref.[32] with the permission from American Physical Society, 2008

of impurity. In Figure 3.12 (b), the variation in conductance with frequency at µ = 0 and T = 0 K is shown. At 300 K temperature and zero chemical potential, the in-plane conductivity of multilayer graphene variation with chemical potential is low. In the beginning, this value suddenly decreases, then it maintains low variation. At the same condition, the perpendicular plane conductivity of multilayer graphene suddenly decreases with frequency. After reaching a minimum value, it increases with frequency, and then the value of perpendicular conductivity reaches a saturation point. In Figure 3.12 (c), the variation in conductivity with frequency at T = 300 K and µ = 0 Ʌ is shown. At T = 0 K and µ = 0.025 Ʌ, the in-plane conductivity of multilayer graphene varies irregularly with frequency for different impurity levels. At the same condition, the perpendicular conductivity of multilayer graphene decreases with frequency, and after a certain value of frequency, the value of perpendicular conductivity of multilayer graphene increases, fnally reaching saturation points. In Figure 3.12 (d), the variation in conductivity of graphene with frequency at T = 0 K and µ = 0.025 Ʌ is shown. At T  =  300 K and µ  =  0.025 Ʌ, the in-plane conductivity of multilayer

34

Emerging 2D Materials and Applications

FIGURE 3.13 (a) Variation in dc conductivity of multilayer graphene with frequency at T = 300 K, µ = 0.025 Ʌ. (b) Variation in dc conductivity of multilayer graphene with temperature at µ = 0 Ʌ, ꙍ = 0. Source: Reproduced from ref.[32] with the permission from American Physical Society, 2008

graphene decreases frequency. After reaching the minimum value of conductivity, it will increase with frequency. Then it reaches a maximum value. With a further increase of frequency, the conductivity decreases again and reaches saturation level. The maximum value and minimum value of in-plane conductivity of multilayer graphene for a low level of impurity is greater than that of a higher level of impurity. At the same condition, the perpendicular conductivity of multilayer graphene decreases with frequency. After reaching the minimum value, it will increase, but after a certain value of frequency, it reaches a saturation point. In Figure 3.13 (a) the variation in conduction of multilayer graphene with frequency at T = 300 K and µ = 0.025 Ʌ is shown. At zero chemical potential and zero frequency, the in-plane dc conductivity for the lower level of impurity will increase with temperature. For a higher level of impurity, the temperature has a low impact on in-plane conductivity. In Figure 3.13 (b) the variation in conductance of multilayer graphene with temperature at µ = 0 Ʌ, ꙍ = 0 is shown. However, the perpendicular dc conductivity of multilayer conductivity increases with temperature. From higher impurity level to lower impurity level, the temperature infuence on outer plane conductivity increases. At µ = 0.025 Ʌ and zero frequency, the in-plane dc conductivity of multilayer conductivity shows weak temperature-dependent for different values of impurity level. For the lower level of impurity, the perpendicular conductivity shows weak temperature-dependent. But for higher impurity levels, it shows no variation. In Figure 3.14, the variation in dc conductivity with temperature at µ = 0.025 Ʌ, ꙍ = 0 has been shown.

3.4.2.3 Variation in the Mobility with Layers Number Graphene is described as an electron gas system having mobility near about 10,000 cm2v−1s−1 [35] at room temperature. The electrical property of graphene on SiO2 substrate varies from 2,000 to 20,000 cm2v−1s−1 [36, 37]. Mobility in graphene depends on the layer numbers [38]. In Figure 3.15 (a) and 3.15 (b), the variation in conductivity

35

Electrical Properties of Graphene

FIGURE 3.14 ꙍ = 0.

Variation in dc conductivity of multilayer with temperature at µ = 0.025 Ʌ,

Source: Reproduced from ref.[32] with the permission from American Physical Society, 2008

(a) @RT

3

2000 4 1000

0 -30

6

10000

-10 0 10 20 Gate voltage / V

30

8 1

0.002

0 -30

8000

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0.004 0.003

Graphene Electrode

@RT

3 6

0.001 8

-20

(c)

0.006

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1

3000

(b)

0.005

2

4000

Conductivity / S

Sheet resistivity / Ohm

5000

-20

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Electrode 10 µm

6000

30

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2

3

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Layer #

FIGURE 3.15 Variation in (a) conductivity and (b) resistivity of different layer graphene with gate voltage. (c) Variation in mobility of charge carrier in graphene with layer numbers. Source: Reproduced from ref.[38] with permission from the Japan Society of Applied Physics, copyright 2009

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Emerging 2D Materials and Applications

and sheet resistance with gate voltage is shown. Here, the four-probe measurement technique was used (because graphene was not decayed by the deposition of metal electrode) [38]. The resistivity of graphene on SiO2 substrate augments with the decrease in layer number. For monolayer, resistivity changes in a drastic manner. In Figure 3.15 (c), the variation in the mobility of charge carrier in graphene with layer number is shown. The carrier mobility in graphene varies with layer numbers. According to this fgure, carrier mobility for monolayer graphene is greater than other layer numbers. But the transport of electrons in monolayer graphene is not ballistic. Electron conductivity of monolayer graphene is decreased by charge impurity scattering [39]. Reduction of interlayer scattering may enhance the current modulation with decreasing layer number. With the decrease of layer number from bilayer to monolayer, the mobility dramatically increases, owing to the variation from the quadratic relationship to linear dispersion relationship [38].

3.5 CONCLUSION To be able to tune the graphene’s electrical conductivity according to need is an astounding progress in the feld of research. In this chapter, we have discussed graphene’s structure. Theoretically, ideal graphene shows extraordinarily high in-plane electrical conductivity. The cause for such high conductivity has been discussed in brief. The electrical conductivity of graphene can be tuned in a number of different ways. The effect of different parameters, like temperature, pressure, external electric feld, doping, defect, etc., on the electrical properties of graphene has been discussed. Ideal graphene is a zero bandgap material. However, through proper treatments, bandgap can be generated in graphene. This generated bandgap can also be tuned according to need. The process of generating bandgap has also been discussed. Finally, the electrical properties of bilayer graphene have been discussed. Proper engineering of this wonder material could cause a revolution in the feld of electronics.

REFERENCES 1. K. S. Novoselov, A. K. Geim, and S. V. Morozov, “Electric feld in atomically thin carbon flms,” Science, 306 (2004) 666–669. https://doi.org/10.1126/science.1102896 2. A. H. Castro Neto, F. Guinea, N. M. R. Peres, K. S. Novoselov, and A. K. Geim, “The electronic properties of graphene,” Rev. Mod. Phys., 81 (2009) 109–162. https://doi. org/10.1103/RevModPhys.81.109 3. B. Guo, L. Fang, B. Zhang, and J. Ru Gong, “Graphene doping: A review,” Insciences J., 1 (2011) 80–89. http://doi.org/10.5640/insc.010280 4. P. R. Wallace, “The band theory of graphite,” Phys. Rev., 71 (1947) 622–634. https://doi. org/10.1103/PhysRev.71.622 5. D. R. Cooper, B. D’Anjou, N. Ghattamaneni, B. Harack, M. Hilke, A. Horth, N. Majlis, M. Massicotte, L. Vandsburger, E. Whiteway, and V. Yu, “Experimental review of graphene,” ISRN Condens. Matter Phys., 2012 (2012) 56. https://doi.org/10.5402/2012/501686 6. S. Das Sarma, S. Adam, E. H. Hwang, and E. Rossi, “Electronic transport in two-dimensional graphene,” Rev. Mod. Phys., 83 (2011) 407. http://doi.org/10.1103/RevModPhys.83.407 7. M. I. Katsnelson, K. S. Novoselov, and A. K. Geim, “Chiral tunnelling and the Klein paradox in graphene,” Nat. Phys., 2 (2006) 620–625. http://doi.org/10.1038/nphys384

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8. J. Schiefele, F. Sols, and F. Guinea, “Temperature dependence of the conductivity of graphene on boron nitride,” Phys. Rev., 85 (2012) 195420. http://doi.org/10.1103/ PhysRevB.85.195420 9. C. H. Ahn, A. Bhattacharya, M. Di Ventra, J. N. Eckstein, C. Daniel Frisbie, M. E. Gershenson, A. M. Goldman, I. H. Inoue, J. Mannhart, A. J. Millis, A. F. Morpurgo, D. Natelson, and J.-M. Triscone, “Electrostatic modifcation of novel materials,” Rev. Mod. Phys., 78 (2006) 1185. http://doi.org/10.1103/RevModPhys.78.1185 10. E. Dorgan, M. Bae, and E. Pop, “Mobility and saturation velocity in graphene on SiO2,” Appl. Phys. Lett., 97 (2010) 082112. http://doi.org/10.1063/1.3483130 11. K. S. Novoselov, V. I. Falko, L. Colombo, P. R. Gellert, M. G. Schwab, and K. Kim, “A roadmap for grapheme,” Nature, 490 (2012) 192–200. http://doi.org/10.1038/nature11458 12. P. Ares, M. Pisarra, P. Segovia, C. Díaz, F. Martín, E. G. Michel, F. Zamora, C. GómezNavarro, and J. G. Herrero, “Tunable graphene electronics with local ultrahigh pressure,” Adv. Funct. Mater., 29 (2019) 1806715. https://doi.org/10.1002/adfm.201806715 13. L. Ci, Li Song, C. Jin, D. Jariwala, D. Wu, Y. Li, A. L. Srivastava, Z. F. Wang, K. Storr, L. Balicas, F. Liu, and P. M. Ajayan, “Atomic layers of hybridized boron nitride and graphene domains,” Nat. Mater., 9 (2010) 430–435. http://doi.org/10.1038/nmat2711 14. J. Kotakoski, A. V. Krasheninnikov, Y. Ma, A. S. Foster, K. Nordlund, and R. M. Nieminen, “B and N ion implantation into carbon nanotubes: Insight from atomistic simulations,” Phys. Rev. B, 71 (2005) 205408. https://doi.org/10.1103/PhysRevB.71.205408 15. T. B. Martins, R. H. Miwa, A. Silva, and A. Fazzi, “Electronic and transport properties of boron-doped graphene nanoribbons,” Phys. Rev. Lett., 98 (2007) 196803. https://doi. org/10.1103/PhysRevLett.98.196803 16. P. A. Denis, “Band gap opening of monolayer and bilayer graphene doped with aluminium, silicon, phosphorus, and sulfur,” Phys. Lett., 492 (2010) 251–257. https://doi. org/10.1016/j.cplett.2010.04.038 17. O. Aktürk, and M. Tomak, “Bismuth doping of grapheme,” Appl. Phys. Lett., 96 (2010) 081914. https://doi.org/10.1063/1.3334723 18. S. Yu, W. Zheng, C. Wang, and Q. Jiang, “Nitrogen/boron doping position dependence of the electronic properties of a triangular graphene,” ACS Nano, 4 (2010) 7619–7629. https://doi.org/10.1021/nn102369r 19. H. Tanaka, R. Arima, M. Fukumori, D. Tanaka, R. Negishi, Y. Kobayashi, S. Kasai, T. K. Yamada, and T. Ogawa, “Method for controlling electrical properties of single-layer graphene nanoribbons via adsorbed planar molecular nanoparticles,” Sci. Rep., 5 (2015) 12341. http://doi.org/10.1038/srep12341 20. T. Shimizu, J. Haruyama, D. C. Marcano, D. V. Kosinkin, J. M. Tour, K. Hirose, and K. Suenaga, “Large intrinsic energy bandgaps in annealed nanotube-derived graphene nanoribbons,” Nat. Nanotechnol., 6 (2011) 45–50. http://doi.org/10.1038/ nnano.2010.249 21. F. Banhart, J. Kotakoski, and A. V. Krasheninnikov, “Structural defects in graphene,” Phys. Rev., 5 (2011) 26–41. http://doi.org/10.1021/nn102598m 22. K. Xu, and P. D. Ye, “Theoretical study of atomic layer deposition reaction mechanism and kinetics for aluminum oxide formation at graphene nanoribbon open edges,” J. Phys. Chem., 114 (2010) 10505–10511. 23. L. G. Cancado, M. A. Pimenta, B. R. A. Neves, M. S. S. Dantas, and A. Jorio, “Infuence of the atomic structure on the raman spectra of graphite edges,” Phys. Rev. Lett., 93 (2004) 247401. https://doi.org/10.1103/PhysRevLett.93.247401 24. A. Orlof, J. Ruseckas, and I. V. Zozoulenko, “Effect of zigzag and armchair edges on the electronic transport in single-layer and bilayer graphene nanoribbons with defects,” Phys. Rev. B, 88 (2013) 125409. http://doi.org/10.1103/PhysRevB.88.125409

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25. K. Wakabayashi, M. Fujita, H. Ajiki, and M. Sigrist, “Electronic and magnetic properties of nanographite ribbons,” Phys. Rev. 59 (1999) 8271. https://doi.org/10.1103/ PhysRevB.59.8271 26. J. Li, I. Martin, M. Buttiker, and A. F. Morpurgo, “Topological origin of subgap conductance in insulating bilayer graphene,” Nat. Phys., 7 (2011) 38. http://doi.org/10.1038/ nphys1822 27. A. Nimbalkar, and H. Kim, “Opportunities and challenges in twisted bilayer graphene,” Nano-Micro Lett., 12 (2020) 126. https://doi.org/10.1007/s40820-020-00464-8 28. J. T. Robinson, S. W. Schmucker, C. B. Diaconescu, J. P. Long, J. C. Culbertson, T. Ohta, A. L. Friedman, and T. E. Beechem, “Electronic hybridization of large-area stacked graphene flms,” ACS Nano, 7 (2013) 637–644. http://doi.org/10.1021/nn304834p 29. Z. Yu, A. Song, L. Sun, Y. Li, L. Gao, H. Peng, T. Ma, Z. Liu, and J. Luo, “Understanding interlayer contact conductance in twisted bilayer graphene,” Small (2019) 1902844. https://doi.org/10.1002/smll.201902844 30. H. Polshyn, M. Yankowitz, S. W. Chen, Y. X. Zhang, K. Watanabe, T. Taniguchi, C. R. Dean, and A. F. Young, “Large linearin-temperature resistivity in twisted bilayer graphene,” Nat. Phys., 15 (2019) 1011–1016. https://doi.org/10.1038/s41567-019-0596-3 31. J. B. Yin, H. Wang, H. Peng, Z. J. Tan, and L. Liao, “Selectively enhanced photocurrent generation in twisted bilayer graphene with van Hove singularity,” Nat. Commun., 7 (2016) 10699. https://doi.org/10.1038/ncomms10699 32. J. Nilsson, A. H. Castro Neto, F. Guinea, and N. M. R. Peres, “Electronic properties of bilayer and multilayer graphene,” Physical Review B, 78 (2008) 045405. https://doi. org/10.1103/PhysRevB.78.045405 33. U. Mogera, S. Walia, B. Bannur, M. Gedda, and G. U. Kulkarni, “Intrinsic nature of graphene revealed in temperature-dependent transport of twisted multilayer grapheme,” J. Phys. Chem. C, 121 (2017) 13938–13943. https://doi.org/10.1021/acs. jpcc.7b04068 34. A. K. Geim, and K. S. Novoselov, “The rise of graphene,” Nat. Mater., 6 (2007) 183–191. http://doi.org/10.1038/nmat1849 35. Y. W. Tan, Y. Zhang, K. Bolotin, Y. Zhao, S. Adam, E. H. Hwang, S. Das Sarma, H. L. Stormer, and P. Kim, “Measurement of scattering rate and minimum conductivity in graphene,” Phys. Rev. Lett., 99 (2007) 246803. https://doi.org/10.1103/PhysRevLett.99.246803 36. C. Jozsa, M. Popinciuc, N. Tombros, H. T. Jonkman, and B. J. van Wees, “Electronic spin drift in graphene feld-effect transistors,” Phys. Rev. Lett., 100 (2008) 236603. https://doi. org/10.1103/PhysRevLett.100.236603 37. K. Nagashio, T. Nishimura, K. Kita, and A. Toriumi, “Mobility variations in mono-and multi-layer graphene flms,” Appl. Phys. Express, 2 (2009) 025003 http://doi.org/10.1143/ apex.2.025003 38. K. Nomura, and A. H. MacDonald, “Quantum hall ferromagnetism in graphene,” Phys. Rev. Lett., 96 (2006) 256602. https://doi.org/10.1103/PhysRevLett.96.256602 39. S. Adam, E. H. Hwang, V. M. Galitski, and S. Das Sarma, “A self-consistent theory for graphene transport,” PNAS, 104 (2007) 18392. https://doi.org/10.1073/pnas.0704772104

4

Electrical Transport in Post-Graphene 2D Materials Ghulam Dastgeer, Sobia Nisar, and Jonghwa Eom

CONTENTS 4.1

Introduction .....................................................................................................39 4.1.1 Transition Metal Dichalcogenides (TMDs).........................................41 4.1.2 Black Phosphorus (BP) ........................................................................43 4.1.3 MXenes as 2D Material .......................................................................45 4.2 Overall View of Post-Graphene 2D Materials................................................ 46 4.2.1 Electronics and Sensing.......................................................................47 4.2.2 Photonic and Optoelectronic Applications ......................................... 48 4.2.3 Power and Energy............................................................................... 48 4.3 Band Structure ................................................................................................ 49 4.4 Doping, Defects, Alloys...................................................................................51 4.5 Electrical Transport .........................................................................................52 4.5.1 Heterostructures.................................................................................. 54 4.5.2 PN Diodes and Homojunctions Devices............................................. 56 4.5.3 High-Frequency Devices .....................................................................59 4.6 Challenges and Opportunities of Post-Graphene 2D Materials ......................62 References................................................................................................................ 63

4.1

INTRODUCTION

Material science and condensed matter physics have disclosed the details about the astonishing applications of graphene as it was cleaved from the graphite chunk by a scotch tape. Graphene is considered an important catalyst in the feld of two-dimensional (2D) materials having exceptional electronic features. Due to its extraordinary electronic spectrum, graphene has prompted the rise of another worldview of relativistic condensed matter physical science. Few of them are unobtrusive in high-energy physical science. In general, graphene is only a single atom thick, which introduces a new category of materials based on two-dimensional physics that provide a large variety of applications.1,2 The study of 2D sheets of graphene has been ongoing for more than a half decade. However, in all those years, researchers were convinced that 2D crystals couldn’t be realized due to their thermodynamic instability. The Peierls and Landau theory, which was further enhanced by Mermin, describes that in a low dimensional crystal lattice, due to the thermal fuctuations at DOI: 10.1201/9781003247890-4

39

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Emerging 2D Materials and Applications

FIGURE 4.1 All dimensions of graphite materials are derived from the single atomic layer thick graphene. The rounded graphene is shaped to 0D buckyballs. The 1D nanorods/ nanotubes are formed by rolling the 2D graphene sheets while stacking several layers over one another to form 3D bulk graphite. Source: (ref.2) Copyright 2007, Nature Publishing Group

any fnite temperature, the atom displacement became comparable to interatomic distance.3 Moreover, as the thickness declines, the melting temperature of the thin flm quickly reduces, which makes the flm unstable.4,5 However, in 2004, graphene was discovered by tearing the graphite pieces over and over again and managed to generate solitary thick atom fakes. Now, we know the ripples in graphene play an important role in the realization of truly 2D materials. Graphene is the wonderful masterpiece of carbon, having a fat monolayer of carbon atoms strongly packed in the 2D honeycomb lattice and is the fundamental building block of graphitic material. It can be enfolded into 0D, rolled into 1D nanotubes, or piled up into 3D graphite.6 In general, all the dimensions of a graphite are based on the single atomic layer of graphene, as shown in Figure 4.1. Yet before exploring the previous work on 2D materials, it is valuable to characterize what 2D crystals are. The 2D crystals are based on a single atomic plane, in which atoms are attached with each other by covalent bonds. In electronics, the number of layers extensively made an impact on the electronic structure. Approximately, hundred layers are favorable to make the thin flm of 3D material. However, in the case of graphene (2D crystals) composed of one or two atomic layers, electronic

Electrical Transport

41

spectra are quite simple. Graphene is narrated as zero energy bandgap or zero-overlap semimetals. For layers three or more, the spectra become complex because the conduction and valance band begin to overlap. Also, various charge carriers become visible. Therefore, graphene one, two, and few (three to less than ten) layers discriminate three diverse types of 2D crystals.7–9 Toward the electronic applications, the most practical and doable way is the epitaxial development of graphene and, with so much in question, fast advancement toward this path is anticipated. Graphene-supported electronic applications grab the attention of various organizations. Intel and IBM centered their research in this direction to keep an eye on possible development. Graphene’s golden era is not unanticipated because now Si-based technology is outdated. Therefore, there must be any new material that can take over the position of Si, and for that, graphene seems to have a potential option. The most instant and pressing application of graphene is found in the utilization of composite material. In electric batteries, the use of graphene powder is another exciting likelihood for the graphite market. It will help in upgrading the effectiveness of batteries. Carbon nanotubes can also be used for this application, but due to its low price of production and being economical, graphene powder has an advantage over the carbon nanotube.10 Moreover, graphene powder can offer even more promising emitting properties, and thin graphite fakes were utilized in the commercial prototype (plasma display). Graphene also provides outstanding material for solid-state gas sensors.11 In addition, hysteretic magnetoresistance,12 and substantial bipolar supercurrents13 are considered as major applications of graphene. The main reason for putting all theoretical and experimental to graphene is due to its unique nature of charge carriers and excellent incomparable electronic traits in isolated form. Schrodinger equation to explain electronic properties of the material is favorable in condensed matter physics. On the other hand, graphene is different, which elucidates with the Dirac equation rather than Schrodinger equation because their charge carrier resembles the relativistic particles.14,15 The researcher, from experience, knows that new exciting dimensions of physics will be revealed after high-quality samples. That’s why the entire focus of the research community is on the graphene. After a series of experiments, it is revealed that the zero-bandgap of the graphene limits its potential for the logic devices, switching applications and photovoltaic devices. This problem has led the researcher to investigate some new class of fexible and bandgap 2D semiconductor materials that could be cleaved in thin sheets like graphene.

4.1.1

TRANSITION METAL DICHALCOGENIDES (TMDS)

The new class of semiconductor 2D materials was the transition metal dichalcogenides (TMDs), which are also atomically thin in nature like graphene. Generally, the TMDs are denoted by a formula of MX2. Where “M” denotes the transition metals like (Mo, W, etc.) and “X” denotes the chalcogen atoms as (S, Se, or Te). In their basic structure, one layer of “M” atoms is surrounded by two layers of “X” atoms. Some TMDs have distinctive structural phases due to transition metallic atoms having dissimilar coordination spheres. Usually, the 2H phase is hexagonal, 1T phase is octahedral, and 1Tʹ phase is known as trigonal prismatic phase. These most regular depicted structural stages are shown in Figure 4.2.

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FIGURE 4.2 The general crystal structure of the single-layer TMDs 2D materials with hexagonal (2H), octahedral (1T), and dimerized (1Tʹ) phases. The in-plane intra-atomic bonding of transition metal and chalcogen atoms is presented for each 2H, 1T, and 1Tʹ phases. Source: (ref.16) Copyright 2017, Nature Publishing Group

The stacking order of these three atoms (X-M-X) defnes the crystal structure and its phases because the 2H and 1T phase are considered thermodynamically more stable as compared to 1Tʹ metastable phase.16 The 2D TMDC semiconductor that gains inclusive focus among all TMDs semiconductor materials is MoS2. It appears as a new class material having astonishing properties.17 MoS2 is one of the most stable 2D TMDC material with a high electron mobility. Each molybdenum atom (Mo) is surrounded by two sulfur atoms (S), and its crystals exist either in 2H phase or 1T phase, as show in Figure 4.3 (a–b). The 2H phase of MoS2 is an excellent and stable n-type semiconductor owing to a large electron density, while the 1T phase is octahedral and shows metallic behavior. MoS2 is a non-centrosymmetric material having a direct energy bandgap of 1.8 eV in its monolayer, while its indirect bandgap is 1.2 eV in case of multilayer MoS2.18–20 The structure of MoS2 is the trigonal prismatic confguration of Mo atoms and hexagonal planes of S. Furthermore, the Raman spectra of MoS2 fakes of different layers are illustrated in Figure 4.3 (c), showing in-plane and outof-plane modes of vibrations. Strong bands at ∼386 and ∼404 cm−1 are the reason for 1 the ascription of E2g (in-plane) and A1g (out-of-plane) SL vibrational methods. SL 1 to ML expansion in the number of layers causes the movement of E2g band toward the lesser wave numbers. In contrast, the band that moves to a higher wave number is 1 A1g . At ∼384 cm−1, the E2g and at ∼406 cm−1, the A1g , BL bands are seen. The ML 1 1 peaks E2g and Ag are noticed at ∼384 and ∼408 cm−1, respectively. The electric transport in few-layer MoS2 device with metal electrodes is presented in Figure 4.3 (d–f). The transfer curve is showing a clear threshold current as the top gate is swept from negative to positive gate voltages, while the inset image is output characteristics of the monolayer MoS2 FET with gold electrodes. The linear trend of the Ids-Vds lines are demonstrating the Ohmic behavior at various gate voltages.23 The electron mobility, which is defned as “how quickly an electron can move in a semiconductor or metal” under the applied electric feld and is mathematically formulated as follows:

˜ FE °

L ˛ dI ds ˙ W ˝˙ dVbg

ˆ 1 ˘ ˘ Cg Vds ˇ

(1.1)

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43

FIGURE 4.3 (A–B) The schematic illustration is showing the crystal structure of 2H and 1T phase of MoS2 crystals, respectively. The Mo atoms are presented with cyan color, while the S atoms are presented in yellow color. (c) The two Raman peaks of MoS2 are showing the in-plane and out-of-plane modes of vibrations, over the Si/SiO2 substrate. (d) The schematic image of the HfO2 covered MoS2 device with gold metal electrodes. (e–f) The back gate dependent transfer curve of monolayer MoS2 sheet and inset image is showing the gate dependent output (Ids-Vds) characteristics. Source: (a–b) (Ref 21) copyright 2020, Inorganic Chemistry Communications; (c) (Ref.22) copyright 2015, Journal of Alloys and Compounds; (e–f) (Ref 23) Copyright 2011, Nature Publishing Group

Where “ µ FE ” is feld effect mobility, “L” is the channel length, “W” is the channel width, “ Cg ” is the gating material/dielectric capacitance, and “Vds” is source-drain dI ds voltage and “ ” is the slope of linear ft in the accumulation region. The linear dVbg trend of the output characteristics at various gates is showing the pure ohmic behavior. This ohmic behavior is attributed to the work function of the Ni-metal and the capping of the MoS2 sheet with tunnel barrier h-BN, which reduces the Schottky barrier height.24,25

4.1.2

BLACK PHOSPHORUS (BP)

Black phosphorus (BP) is another attractive 2D semiconductor material with a small bandgap. It consists of a single layer of phosphorus atoms and considered a crystalline material. The atoms are contented with each other to form an armchair structure,26 as shown in Figure 4.4 (a). The high-resolution transmission electron microscopic

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Emerging 2D Materials and Applications

FIGURE 4.4 (a) Crystal structure of BP sheet demonstrating the armchair structure. The distance between two consecutive layers is equal to ~5.3 A°. (b) High-resolution transmission electron microscopic image of few-layers thin sheet of BP is showing the atomic planes clearly. (c) The electron diffraction pattern of the BP fake. (d) The resistivity ρxx of the BP is plotted as a function of temperatures at different values of the applied magnetic felds in perpendicular direction, ranging from 300 K to 10 K. (e) The normalized magneto resistance of BP as a function of magnetic feld H at various temperatures. (f) The temperature versus crossover magnetic feld plot. The inset graph shows the variation in MR as a function of magnetic feld. Source: (ref 30) Copyright 2016, Nature Publishing Group

HTEM scan of a few-layers thin sheet of BP and its electron diffraction pattern is presented in Figure 4.4 (b–c). Intrinsically, BP is a p-type semiconductor material in nature whose bandgap varies from 0.3 eV ~ 0.2 eV, which depends upon its sheet thickness.27–29 In monolayer BP, large hole mobility (10,000 cm2/Vs) is predicted29 theoretically, which makes it more promising to use in the electronic device. The anisotropic magneto-transport through the BP plays a vital role in future magneto-electronic devices. The sheet dependent resistivity ρxx of BP is plotted as a function of temperature at several magnetic felds. The temperature was elevated from 10 K to 300 K, while the values of perpendicular magnetic feld were changed from 0T to 7T, as shown in Figure 4.4 (d). It is observed that the resistivity decreases abruptly up to 60 K, while above this temperature, BP showed the metallic behavior. This transition is attributed to the thermal activation of the impurities that act like donors. These impurity donors freeze as the temperature is decreased further, and BP shows a signifcant variation in its resistivity below 60 K. The inset of Figure 4.4 (d) is showing the variation in magneto-resistance (MR) as a function of temperature at various magnetic felds. Generally, the MR is described as [ρ(H)  −  ρ(0)]  /  ρ(0)]  ×  100%, where ρ(0) and ρ(H) are the values of resistivity at zero feld and H feld, respectively. The highest value of MR was observed up to 510% at a temperature of 30 K under the effect of 7T magnetic feld.30 Moreover, the felddependent MR is plotted at various temperatures, as shown in Figure 4.4 (e–f). Under

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a threshold value of the applied magnetic feld BL (6 T) at room temperature, the MR is demonstrating a quadratic increase, as shown in Figure 4.4 (f), and then shows a linearly increasing trend with the increasing magnetic feld without saturation.

4.1.3

MXENES AS 2D MATERIAL

A newly emerging and attention seeking material has been discovered from the group of transition metal carbides, nitrides, and carbonitrides, known as MXene. Generally, the MXenes are synthesized from MAX phases comprising of layered ternary carbides, represented with the formula Mn+1AXn. Here M is denoting an early transition metal (Sc, Ti, Nb, Mo, V, Cr, Zr, Hf, or Ta), A is denoting an element that belongs to groups 12–16 (Cd, Al, Ga, Ge, Si, P, S, As, In, Sn, Tl, Pb, or S), and X could be carbon and/or nitrogen. MXenes are usually produced by the top-down synthesis methodology by careful etching of A elements from the MAX phase structure, using reactive acids and exfoliation technique. In the subsequent Mn+1AXn phases, the powerful bonding between M and A elements is substituted by hydrogen bond (e.g., OH, F, or O after replacement of element A), at room temperature. The aqueous solution of HF is used as an etching agent.31–33 All established MAX phases having P63/mmc symmetry are almost layered hexagonal, and the M layers are considered almost closely packed while the X atoms occupied the octahedral vacancies. In general, all the MAX phases are similar to the 2D layered materials, in which carbides and/or nitrides are bonded with an element A, as shown in Figure 4.5. The M-X bond could be a covalent/metallic/ionic bond much stronger, though the M-A bond is metallic.31,34

FIGURE 4.5 Schematic demonstration of the structure of various MAX phases and the subsequent MXenes. Source: Reprinted with permission from ref31, copyright 2014, Wiley

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Emerging 2D Materials and Applications

The band structure of the resultant MXenes calculated by frst-principles densityfunctional theory is typically different than its parent MAX phase. The functionalization of MXenes with F, O, and OH is the promising way to shift it from metallic phase to semiconductor. The work function and bandgap of MXenes could be modulated by surface engineering technique (e.g., the work function of the Ti3C2Tx MXene could be tuned from 6.15 eV to 1.81 eV by terminating the entire O or OH from the surface, respectively). The tunable bandgap and its phase shift from metallic to semiconductor makes MXene a promising candidate for electric and optoelectronic devices. Its sheet resistance varies from 0.1 to 8 KΩ per square with a transmittance effciency up to 90%, which are considered quite good for electric and optoelectronic applications.35,36 This high transmittance and fexible nature of the MXene sheets can play a vital role to fabricate the high effciency solar cells, photodetectors, light emitter diodes, and liquid crystal displays (LCDs).37 After modulating the work function of MXenes, it has been also deployed to form electrodes in thin flm transistors, FETs, and logic devices.38,39 Including a variety of applications, recently MXenes have obtained substantial interest for energy harvesting, energy conversion, and energy storage tools, such as batteries, supercapacitors, and water splitting, because of its excellent physicochemical features of high electric conductivity, high melting point, hardness, and high thermal conductivity and oxidation resistance as well.32,33,40,41

4.2

OVERALL VIEW OF POST-GRAPHENE 2D MATERIALS

The presence of 15 elemental main group 2D materials has been tentatively confrmed or hypothetically anticipated until this point in time, with an assortment of captivating and valuable applications. Only the chemical composition of elements is not enough to describe the material properties of 2D materials, but allotropes are also considered as a strong ally to dictate the optical, electronic, and thermal properties. The electronic attributes of 2D elements in Group IV resemble carbon, the lightest elements of the group. Moreover, Si, Ge, and Sn show the hybridization state between sp2 and sp3, which contains buckled crystal lattice structure like graphene instead of a planner form.42 Group III elements containing 2D aluminum, which is known as aluminene, and 2D boron that is recognized as borophene have various allotropes, in theory, with a bit complicated multi-atomic unit cell. In Group III elements, a lower number of valance electrons might be linked with this complication, ensuing their exclusive electronic band structure.43 The transition from metallic to semimetal or semiconducting actions of monolayer 2D materials is experienced as we move from Group III to Group VI in a periodic table. For example, Group III elements like aluminene, gallenene, and borophene are considered metals. Similarly, Group IV 2D materials attain Dirac cone semimetal band structure like graphene. Moreover, Group IV 2D analogues have spin-orbit coupling, which, with the increase of atomic number, creates a small bandgap around the Dirac cone.44,45 In Group IV, V, and VI, elements like plumbene, bismuthene, selenene, and tellurene also illustrate large spin-orbit coupling.46–49 2D materials from Group IV elements also have several magnetic

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characteristics that are theoretically forecasted. For example, at Curie temperature, Tc = 122 K and 145 k semi-hydrogenated silicone and germanene show the behavior of ferromagnetic semiconductors.50 In nanoscale materials, bandgaps assume an essential part that plays a key role in optoelectronic properties and directs their possible future applications. Therefore, several techniques have been adopted in earlier work, like the electric feld, strain, edge, doping, surface passivation, and functionalization to modulate the bandgap of elemental 2D materials and to bring new and thrilling applications. 2D materials illustrate the unique and outstanding properties in the feld of electronics and sensing, power, and energy. Also, it provides future advancements in photonic and optoelectronic applications.

4.2.1 ELECTRONICS AND SENSING The novel and outstanding properties of elemental 2D materials are ready to empower future headways in electronics and sensing for silicon gadgets. Large numbers of the elemental 2D materials show proftable properties, including high electron and hole mobilities and on/off ratios, topological insulating conduct, material adaptability, and high sensitivities to adsorbed particles that can develop the present state of the art system. Fabrication and material measurement during FETs operation are the conventional means to assess the electrical properties of 2D materials. Until this point in time, active elemental 2D materials with feld-effect transistor (FET) is synthesized using silicene, selenene, phosphorene, and tellurene. All these elements contain a large range of on/off ratios and mobilities. In 2017, exfoliated crystals were utilized to fabricate the transistors of tellurene and selenene. Selenene gadgets with 16 nm thickness showed generally low mobilities in contrast with other basic 2D materials (0.26 cm 2 V−1 s−1) and low on current (20 mA mm−1), however a high on/off proportion ≈10 6 at room temperature.51 Where the on/off ratio is essential, for logic-type apparatus, this ratio might be signifcant to guarantee low current leakage. Thickness-based electrical execution of tellurene crystals, in contrast, a notable and extensive electronic performance is displayed. Moreover, thickness-based optical properties are not shown by typical layered 2D materials.52 Until today, in 2D materials, the future of the electronic and sensing platform is based on three main areas of concentration (fexible electronics, sensors, and spintronics). 2D materials seem to be a strong candidate for future electronics and sensors as they display various stimulating properties; moreover, these materials can show further improved behavior if these materials combine in the form of heterostructure instead of being individual. For example, borophene/perylenetetra carboxylic dianhydride (PTCDA) recently was used for the experimental lateral heterostructure; moreover, abrupt borophene/polymer interface is the reason why these heterostructures exhibit metal/semiconductor conduct and resistive switching.53 In borophene/ PTCDA heterostructure, the evolution in the density of states happens at the molecular length scale due to these sharp interfaces.

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Emerging 2D Materials and Applications

4.2.2 PHOTONIC AND OPTOELECTRONIC APPLICATIONS The plasmonic, photodetector, and ultrafast lasers are key areas of photonics and optoelectronics where 2D materials are deeply explored. Plasmons are aggregate charge oscillations that happen in solids through Coulomb interactions. While in semiconducting 2D material like graphene and phosphorene, plasmons show up in the mid/close infrared frequency range, plasmons in borophene can reach out into the visible range because of their higher carrier concentrations.43 In an anisotropic plasmonic gadget wherein the appliance reaction strain or light polarization induce charges, phosphorene-based devices are used.54 Another rising application of 2D material involving plasmons is thermoplasmonics, where localized hotspots can create using surface plasmons coupled with light irradiation. This technique is highly useful in the medical feld and nowadays successfully utilizing for removing tumors in mice.55 Photodetection is one of the earliest applications of optoelectronics for 2Dmaterials. Bandgaps of 2D semiconducting materials going from visible to infrared frequencies, making them unmistakably appropriate for broadband photodetection.56–58 Current photodetection innovation is fundamentally founded on silicon with its elite performance, maturity, low-cost, and high level of integration with electronics. Perhaps the most intensely read natural materials for photodetectors is phosphorene. Because of the trouble in synthesizing elemental 2D material, for example, borophene, silicene, germanene, what’s more, stanene, their adequacy as photodetector materials remains to be considered and further investigated. In ultrafast laser (fber-based), continuous wavelength laser is being converted into pulsed laser using mode-locking technology. This is accomplished by adopting SA (saturable absorber), a nonlinear optical material whose light absorption is inversely proportional to intensity. Whereas, the ideal SA has good absorption properties. Phosphorene, one of the primary elemental 2D material, was studied as an SA.

4.2.3 POWER AND ENERGY Global communities desire strategies that are reliable, vigorous, and exceedingly profcient for the deliverance, storage, and transmission of power in upcoming years. Batteries are the key component in future electronic devices as they enhance gravimetric and volumetric storage abilities. From in power perspective, 2D materials are considered as a strong entity for next-generation energy devices. Among 2D materials, borophene holds the maximum theoretical capacity for Li.59 For energy storage, rechargeable batteries are the highest priority because of their high energy densities. However, lower power density also occurs due to slow charging and discharging time and short life cycle. Therefore, as an alternate, ultracapacitor or supercapacitor offers super lifetimes, fast charge/discharge rate, and higher power densities. Moreover, phosphorene was considered as one of the prior materials among 2D materials for utilization in supercapacitors.60 Also, elemental 2D materials present a signifcant role in the creation and formation of novel thermoelectric gadgets. However, after loads of applications of 2D materials in the feld of electronics until this point in time,

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still, technological uprising requires a considerable amount of study and exploration of 2D materials in the coming years.

4.3 BAND STRUCTURE Nanodevices that are based on 2D materials instead of conventional semiconductors have been broadly researched and show extremely competitive performance. However, there are extraordinary diffculties to be conquered before fguring out its practical applications. According to the physics of semiconductors, the Fermi level in n-type 2D semiconductor materials exists near the conduction band, while in p-type materials, it lies near the valance band. In 2D gadgets, one of the vital components that direct the charge carrier from metal to 2D channel is the “contact” among metal electrode and semiconductor.61 In deciding the Schottky barrier (SB) that prompts non-negligible contact opposition and accordingly restricts gadget performance, the major and leading part is played by the energy level alignments (ELAs) at the 2D metal surface.62 For instance, a huge SB or contact resistance can essentially break down the performance of a 2D FET, for example, current on/off proportion, feldimpact mobility, and subthreshold swing (SS). Therefore, it is signifcant to create successful contact designing ways to form ohmic contacts in 2D apparatus. Conventional devices that are Si supported to face the unwanted interfacial impacts, for example, Fermi level pinning (FLP) due to the existence of significant dangling bonds on the surface of Si and hence control the confguration of ohmic contacts.63 2D materials with an atomically fat surface unlike bulk Si give an extraordinary chance to form ohmic contacts with metals. However, the crystal structure of 2D materials with the thin atomic nature can be harmed during standard device fabrication procedure, for example, e-beam lithography (EBL) and physical vapor deposition (PVD) of metals, which brings about huge SB or interfacial states at 2D/metal contacts. Furthermore, for accomplishing ohmic contacts, the substitutional doping methods are broadly utilized. For instance, thermal diffusion and ion implantation are not relevant to 2D material because of the introduction of critical defects and annihilation of the crystal lattice. Different methodologies have been adopted to set up van der Waals (vdWs) 2D/metal contacts; for example, to preserve the 2D crystal, mechanical transfer of metal layers was processed to eliminate interfacial residue during device fabrication, thus making a deformity-free contact and removing interfacial states or impacts. In 2D materials, top contact and edge contact are two types of contact geometry. By placing metals on top of 2D material, top contact is created. On the other hand, due to the limited edge area, it is diffcult to attain good edge contact. Heterostructure, according to band alignment, is separated into three major categories recognized as symmetric, staggered, and broken as type I, type II, and type III, respectively, as shown in Figure 4.6 (a–c). In type I, the valance and conduction bands of one material lie within the range of other semiconductor material, while in type II, the bands are shifted within bandgap size. In type III heterostructures, the energy bands of both materials exist out of range with respect to each other. Each band arrangement has specifc applications to empower various assortments of gadgets. In optical devices such as LEDs

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FIGURE 4.6 Three basic types of the energy band diagrams based on various heterostructures. (a) The straddling band alignment is also known as type I. (b) The staggered band alignment is called type II. (c) The broken bandgap, or Type III band alignment. Source: (ref64) Copyright 2013, Chemical Society Reviews

(light-emitting diodes), the most generally used band alignment is type I. Moreover, type I band alignment is utilized in lasers as they give away to spatially restrict electrons and holes, so effective recombination can happen. The capacity to create single and multiple quantum wells has additionally improved the performance of lasers and LED appliances.65 For unipolar electronic tool applications, band alignment, which is considered very valuable, is type II because it has exceptionally strong carrier confnement as they let larger offset on one side (either conduction or valance band). An excellent example of strong carrier confnement in type II is the HEMTs (high electron mobility transistors) based InAs/AlSb quantum.66 In bipolar transistors, as well in quantum well resonant tunneling bipolar transistors,67 conduction band levels can be adopted as hot electron injectors. To design the conduction to valance band transition energy, the heterostructure of type II and type III are additionally helpful. This is especially signifcant in TFETs (tunneling feld-effect transistors) to upgrade the tunneling current density,68 also plays an important role in wavelength photodetector69 and infrared inter-sub band superlattice lasers. Probably the greatest gadgets empowered by semiconductor heterostructures are quantum cascade lasers, which utilize complex heterostructure stacks to design minibands and intra-sub band changes, which permit profcient light emanation from the mid-infrared to the terahertz systems.70 Utilizing 2D materials, numerous gadget ideas have additionally been proposed or acknowledged. To display current rectifcation and compilation of photoexcited carriers, recently, the exhibition of atomically thin p-n diodes of type II heterostructure has been done.71 The latter can additionally be accomplished utilizing graphene/TMD/graphene heterostructures. Type I heterostructure that contains LEDs with the TMD sandwiched among BN and graphene as contacts can also be designed across a wide spectral range.72 In the recent past, type II heterostructures have likewise been investigated as a platform for attaching long-lived interlayer (or indirect) excitons.73 Moreover, for different applications, lateral heterostructures of

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TMDs with different classes of monolayer materials can be utilized. Therefore, in semiconductor heterostructures, the relative band alignment is one of the paramount design parameters because it discovers the type of the heterostructure.

4.4

DOPING, DEFECTS, ALLOYS

Intrinsically the semiconductor materials are either p-type (having holes as majority carriers) or n-type (having electrons as majority charge carriers), but these semiconductors could be modifed according to our desires to develop the desired charge transport. In case of silicon, the phosphorus doping make it n-type semiconductor, while the addition of boron atoms doped the silicon as a p-type semiconductor. The doping of few atoms of magnesium to gallium nitride was also used to build an LEDemitting blue light.74 The discovery of graphene and other bandgaped 2D materials opened the door to develop the smart electric and optoelectronic devices even for the quantum applications. A lot of techniques had been introduced to enhance the performance of the devices composed of 2D materials (e.g., mobility, carrier density and bandgap modulation by surface passivation, high k-dielectric substrate, different metal contacts, and doping).27,76,77 There are various doping techniques of the 2D materials, such as photoinduced doping by DUV light in different gas environments, using dissimilar metals of high or low work functions to dop them as a p-type or n-type semiconductors, and chemical doping by different chemicals.75, 77–79 One of the effective techniques to enhance the device and modulate the charge carrier density from electrons to holes and vice versa is chemical doping. This chemical doping of 2D materials could change the Fermi level and may also increase or decrease the bandgap of the pristine materials. Usually, the p-type doping of the 2D materials shifts their Fermi level near the valance band maxima, while n-type doping shifts the Fermi level near the conduction band maxima. Hua-Min Li et al. introduced the chemical doping of few-layer thick 2D sheets of MoS2 to fabricate a vertical p-n junction with symmetric metal contacts.75 The one surface of the MoS2 sheet was doped to n-type semiconductor by introducing the benzyl-viologen (BV), while the other surface was doped to p-type semiconductor by using gold chloride (AuCl3), as shown in Figure 4.7 (a–d). The electric transport through this chemically doped vertical p-n junction of MoS2 sheets was controlled by back-gate voltage, and an ideality factor of 1.6 was extracted at the positive gate voltage of +60V. Additionally, the photo response and tunneling behavior of chemically doped MoS2 p-n junction was also studied with varying its layer thickness. These characteristics of the chemically doped MoS2 p-n junction were showing a remarkable effect of chemical doping of the 2D materials to build a highly effcient logic devices, p-n junctions, solar cells, and photodetectors. Furthermore, the performance of the 2D materials based devices may be affected because of defects in 2D TMDs sheets. These defects modulate the optical, chemical, magnetic, and electrical properties of the 2D materials and their devices.80 Typically, these defects could occur during the growth of 2D materials, chemical doping, environmental effects, or stability of the 2D materials. The 2D materials have two possible polytypes 2H phase, and 1T semimetallic phase. This 1T phase is not as stable as 2H, and it is not considered as a semiconductor. Secondly, the surface defects

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FIGURE 4.7 Chemically doped few-layers thick MoS2 sheet to fabricate a vertical p-n junction. (a) The one side of the MoS2 sheet was transferred over the PMMA/PVA/Si substrate, and it is n-doped by BV. (b) The n-doped sheet of MoS2 was transferred to a PDMS/glass stamp. (c) The BV doped MoS2 sheet was stacked over the Cr/Pd/Cr electrode and SiO2/Si substrate. (d) The top surface of MoS2 sheet was doped to p-type semiconductor with AuCl3, and a Cr/Pd electrode was deposed over it as a top electrode.75 Source: Copyright 2015, Nature Publishing Group

in the sheets of 2D materials may also arise because of oxidation or inappropriate growth.81,82 It is also known as zero defect, in which some vacancies could be observed within the atomic planes. These defects could degrade the performance of the 2D material for the electric or optoelectronic applications. One of the most environmentally unstable material is black phosphorus (BP), which is oxidized rapidly. Its monolayer fakes could not be deal in air or ambient environment. After exposing the thick fakes of BP in ambient environment, it is observed that its layers are etched gradually up to monolayer and reduced the device performance signifcantly.27,84,85 These defects generated because oxidation could be controlled by passivation of some insulator oxide flms or 2D sheets of h-BN.27,85

4.5

ELECTRICAL TRANSPORT

In 2D semiconductors, Schottky barrier height (SBH) is the key component that charge inoculation highly depends on. To optimize the procedure, the familiarity of its value and information about how to amend it is necessary to know. Schottky-Mott

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rule is referred to as the difference qφB0 = q(φm  − χ) among metal’s work function (qφm) and electron affnity (qχ) of semiconductor. This is how, in the best-case scenario, the SBH (qφB0) between metal and semiconductor is decided. However, in a real situation, the pinning of Fermi level at the metal/semiconductor interface occurs. This can be verifed by checking the dependence of Schottky barrier height on φm, given by S = dφB0/dφm, with S = 1 equivalent to the best case or Schottky limit, and S  ≈  0 related to that of a pinned Fermi level. The existence of metal-induced gap states is the reason behind the metal/bulk semiconductor pinning.63 In thermionic emission, the measurement of activation energy is the generic methodology of extracting the Schottky barrier height. At the source and drain, two Schottky barriers are linked back-to-back, respectively, in the geometry of SB FET. However, the consumption of voltage drop goes to the maximum in the reversebiased contact. Therefore, transistor behavior dominates. The following equation represents the current density injection through a reverse-biased SB: ˛ q ˆ ˛  qV J ˜ A*T  exp ˙ ° B0 ˘ ˙1 ° exp  ° k T B ˇ˙ ˝  kBT ˝

ˆ ˘  ˘ˇ

(1)

Where A* represents the Richardson constant that can be derived for semiconductors of 2D and 3D. SBH shows with the qϕB0 , exponential characterized by α which is equal to 2 and 3/2 for SCs and 2D SCs, respectively, T is temperature, V stands for applied bias, and k B is the Boltzmann constant. Moreover, for qV >> k BT, equation (1) is modifed to: ˛ q ˆ J ˜ A*T  exp ˙ ° B0 ˘ ˝ k BT ˇ

(2)

If the transistor conduct in the subthreshold regime (small gate voltage Vg). Therefore, the previous equation again is simplifed as follows: ˛ E ˆ J ˜ A*T  exp ˙ ° A ˘ ˝ k BT ˇ

(3)

Where E A ˜ q˝ B0 ° EC˙ ˛ ECO is the total activation energy that charge carriers must trounce to access the channel, whereas EC˜ ° ECO is the difference between the conduction band minimum in the bulk and at the interface. Schottky barrier and activation energy are not a similar thing. Unlike all values of Vg, when the fat band state is met, it is possible to identify SB and activation energy. The channel opposition plays an important role in the observed temperature reliance. With decrease of temperature, the resistance goes down when the MoS2 channel is gated further than the metal-insulator transition. Also, the overall opposition (channel plus contact) can then decrease, but it cannot misconstrue as a negative SB. The SBH is required to rely unequivocally upon the number of layers because the bandgap rises as the thickness is reduced in the restriction of one to three layers. From 2 nm to multilayers, there is no huge reliance on the SBH on MoS2 thickness. It is experienced that if the Fermi level is pinned to around 100 meV beneath the

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TABLE 4.1 Performance Summary of Different Metal Electrodes Fabricated over MoS2 Sheet87 Metals Al W Au Pt

Work Function, φM (eV) 4.5 5.1 5.4 5.7

Schottky Barrier Height, φB = φM – X (eV) 0.5 1.1 1.4 1.7

conduction band, then the SBH relies feebly upon the metal work (S = 0.09). This is in inconsistency with early outcomes from X-beam photoelectron spectroscopy estimations, which showed that the SBH for bulk MoS2 is in the Schottky limit (S = 1).86 Four diverse metals were carefully considered with varying work functions to investigate the properties of metal when it comes in contact with nanostructured flms of 2D MoS2 nanofakes. These metals include Al, Pt, W, Au. Comparative to the X = 4.0 eV that is electron affnity of MoS2, Au, and Pt with ˛ ˜Au ° 5.40eV ˝ and ˛ ˜Pt ° 5.70eV ˝ were opted as a high work function while as a low function, metals were Al and W with ˛ ˜Al ° 4.54eV ˝ and ˛ ˜W ° 5.09eV ˝ . Under identical bias conditions, Al/MoS2 and Pt/MoS2 interface reveals the highest and lowest current, respectively. The experimental outcome attained utilizing various metal contacts with varying functions are recapitulated in Table 4.1. It indicates that Al (the lowest work function metal) has the highest bias current, representing its better ability of charge injection at room temperature for an interface with MoS2 in contrast to Au and Pt. The estimated SBH by the Schottky-Mott affliation is also publicized in Table 4.1.87 For infusion of electron (n-type) and hole (p-type), ohmic contacts are acknowledged through n+/p/n+ or p+/n/p+ substitutional/debasement doping profles, respectively, in traditional Si FETs. In any case, early showings of 2D FETs without such controllable and supportable doping plans depended on the utilization of elemental metals with various work capacities for carrier infusion into the respective band of the 2D channel. There has been some advancement in doping 2D materials; however, these procedures are generally surface electrostatic doping, where a charged species are generated on a superfcial level outcome in the gathering of electrons or hole openings in the semiconductor

4.5.1

HETEROSTRUCTURES

In previous few years, the novelty and versatility of TMDCs (WS2, MoS2, MoSe2, etc.) and 2D materials like hexagonal boron nitrides (h-BN) and graphene have attracted considerable attention. Figure 4.8 (a) illustrates the evolution from semimetallic graphene to insulator h-BN 2D materials, which are possibly cleaved to get the required layer numbers. The graphene is a 2D material with zero bandgap while the

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FIGURE 4.8 (a) The evolution of 2D materials from semi metallic graphene to insulator h-BN. (b–f) Schematic representation of 2D materials and various type of their heterostructures with 0D, 1D, 2D, and 3D materials. Source: (ref.89). Copyright 2016, Nature Publishing Group

2D semiconductors have a reasonable bandgap varying from 0.3 eV to 2eV and h-BN is an insulator 2D material with a large bandgap of 6.08 eV.1,27,28,88,89 The modifed 2D layers and their integration into lateral and 2D heterostructure provides many more opportunities to fabricate switching devices, photo detectors, photovoltaic cells, and high frequency devices.27,79,90 The hybrid structure of the operating artifcial 2D provides a paragon of innovative applications and properties beyond their component 2D atomic crystals. Therefore, this is considered as a novel and exciting feld of progressive research. The 2D materials based heterostructure could be categorized to, e.g., 0D quantum dots, 1D nano rods/nanowires, 2D sheets, and 3D stacking of bulk materials over the 2D material presented in Figure 4.8 (b–f). The 0D-2D heterostructure are usually based on the 2D materials decorated with nanoparticles or quantum dots, while the 1D-2D heterostructures are based on 2D material and nano-rods/nanowires. These two formerly mentioned heterostructures are considered very promising to fabricate the fast and broadband photodetectors and atomically thin FETs with switching applications. Each heterostructure has its unique characteristic that can be used for various applications (e.g., power conversion effciency, p-n diodes, photovoltaic applications, energy harvesting, and data storage).27,91–93 The most intriguing applications could be attained by 2D-2D atomically thin van der Waals heterostructure.94,95 In optoelectronics and electronics, a comprehensive scope of bandgap-engineered applications has been provided by this heterostructure strategy.96 In vertical stacking, weak interlayer van der Waals forces and strong intralayer covalent bonding has been present in layered materials.97 To make atomic-layered 2D materials, several schemes have been anticipated thus far. In experimental

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and theoretical work, many vertical heterostructure such as TMDC/TMDC stack, h-BN/Gr stack, Gr/h-BN stack, and Gr/TMDC stack has been investigated. Most of stacks shows a huge potential for various applications.98–100 Different technologies were recommended for the formation of artifcial 2D layers, from manipulation, transfer, and stacking of layers. Moreover, to achieve a high standard vertical heterostructure, mechanical exfoliated 2D layers could be employed. The 2D-2D van der Waals heterostructures are getting attention to fabricate the p-n junctions, homo junctions, n-p-n (p-n-p) transistors, photovoltaic devices, and solar cells.27,28 Recently, TMDs and graphene-based heterostructures are also showing promising results to fabricate the spintronic devices. The stacking of few layers of TMDs over the graphene increases the spin orbit interaction of graphene up to several millielectron volts, which makes it more effcient to control the spin transport through it.101,102

4.5.2

PN DIODES AND HOMOJUNCTIONS DEVICES

The major building block of integrated circuits (ICs) is the p-n junction diode, which consists of two different types of semiconductor materials. The combination of a p-type semiconductor material and n-type semiconductor material establishes a p-n junction, which is also known as a diode. These diodes are used as rectifers for the rectifcation purposes, logic switches, high frequency, and logic devices and many more applications. As the thin and fexible 2D materials (e.g., WS2, WSe2 MoS2, MoTe2, BP, etc.) are discovered, the van der Waals heterojunction attained a huge attention to develop the p-n junctions, photovoltaic cells, solar cells, and the optoelectronic devices. A thin sheet of the p-type 2D material is stacked over the n-type 2D material, which results in a van der Waals p-n heterojunction. The diodes composed of thin sheets of 2D materials are classifed into three major groups. The frst class is known as lateral p-n junctions, in which a p-type TMDC sheet is attached with the n-type TMDS sheet or n-type thin oxide flm in a plan. Recently, Deng, Y. et al. reported103 a lateral van der Waals diode composed of n-type monolayer MoS2 sheet and p-type black phosphorus sheet. Another combination of thin 2D materials and semiconducting oxide flms were also used for the fabrication of p-n diode, as shown in Figure 4.9 (a). A thin sheet of 2D semiconducting p-type black phosphorus and n-type metal oxide indium gallium zinc oxide (n-IGZO) was combined together to form a van der Waals p-n junction. The electric transport through such diodes may arise because of the sharp interface between p-type and n-type materials, metal electrodes, carrier concentration, and electrostatic gating.79,104 Using a gate voltage, the carrier transport and rectifcation ratio could be modulated through the lateral p-n junctions, as shown in Figure 4.9 (b–c). This lateral van der Waals p-n junctions are considered best candidates to get a large rectifcation ratio up to 106, and for the effcient photovoltaic applications.27,28,103 The second class of the diodes is the vertical diodes. The vertical diodes are composed of the single or multilayer sheets of 2D TMDS materials of different nature, sandwiched between two metal contacts. The p-i-n diodes also belong to this class of diodes, as shown in Figure 4.10 (a–b). The WSe2 sheets of various thickness varying from monolayer to 72 layers were inserted in between Gd metal and Pt metal electrodes. The Gd is a low work function metal (ΦGd ≈ 2.9 eV) which doped the

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FIGURE 4.9 (a) The schematic diagram of IGZO/p-BP van der Waals diode. (b) The ionic liquid gate dependent logarithmic I-V curves are plotted as a function of source-drain biasing voltage. Inset is showing the linear I-V curves at various gate voltages. (c) The gate-dependent rectifcation in IGZO/p-BP van der Waals diode. Source: (adapted from27) Copyright 2019, American Chemical Society

few layers of WSe2 to more n-type, while because of the high work of Pt (ΦPt ≈ 5.6 eV), it doped WSe2 to more p-type. Such asymmetric metal electrodes are providing an excellent choice to make vertical p-i-n diodes, in which rectifcation ratio could be modulated by changing their thickness. The thickness dependent I-V curves and rectifcation ratio are depicted in Figure 4.10 (c–d). The electric transport through the vertical diodes based on the tunneling through the 2D material, its thickness, and metal electrodes of different work functions.27,79 Increasing the thickness of the 2D semiconducting sheets, the rectifcation ratio increases. Even in vertical diodes, a series of quantum tunneling occur such as direct tunneling (DT), Fowler-Nordheim tunneling (FN), and Schottky emission tunneling (SE), and these tunneling also play a major role to enhance the rectifcation ratio. The band alignment and various types of the tunneling are presented in Figure 4.10 (a–c). The latterly mentioned two effects (DT and FT) are because of electrons and holes tunneling through the metal and 2D semiconductor sheets junction, while the

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FIGURE 4.10 (a–b) The schematic images of the vertically stacked Gd/WSe2/Pt device. (c) The output characteristics of the vertical p-i-n diode with varying the thickness of WSe2 sheet from monolayer to 72 layers. (d) At room temperature, the highest rectifcation ratio was attained with the 64 layers thick WSe2 sheet, sandwiched between Gd and Pt metal electrodes. (e) The band alignment and electric charge transport because of a series of quantum tunneling is presented through a Gd/WSe2/Pt based p-i-n heterojunctions. The energy band alignment is presented at equilibrium (e) with zero biased voltage VSD = 0V. (f) Tunneling mechanism and charge transport during the forward biasing and (g) during the reverse biasing when VSD < 0V. Source: (ref.79) Copyright 2018, Nature Publishing Group

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former tunneling (SE) mechanism is responsible of the energy bands of the insulating layers. In contrast, when the vertical diodes are reverse biased, then the electron and holes transport via diodes are entirely dependent on the DT and FN tunneling, but the SE tunneling will be negligible because of the large Schottky barriers. The vertical diodes are used for the high frequency devices, logic switching, and tunneling applications. The third class of the p-n diodes is the homojunctions p-n diodes. In homojunction devices, one-half of the 2D semiconductor material’s sheet is doped as a p-type semiconductor, while the rest of the sheet is doped as an n-type semiconductor.105 The same 2D material possessed both types of the charge carriers (holes and electrons). This doping concentration can also be controlled by electrostatic gating.106 Recently, Andreas, P. et al. reported a WSe2 monolayer p-n diode106 in which charge transportation was controlled by using two bottom metal gates. A monolayer WSe2 sheet was transferred over the silicon nitride, having two bottom metal electrodes as gating electrodes. Finally, they fxed the one bottom gate at VG1 = −40 V, and the other bottom gate was fxed to VG2 = +40 V. This gating technique (VG1  0 V) tuned the one half of the monolayer WSe2 to p-type by inducing the holes as majority carriers, and the other half part of monolayer WSe2 was doped to n-type semiconductor by inducing electrons as majority carriers, as shown in Figure 4.11 (a–c). The half part of the WSe2 sheet over the VG10V, will behave as n-type WSe2. In this technique, the same fake is turned into a homojunction p-n diode, and the carrier transport is modulated by gate voltages. In contrast, there are several reports based on chemical doping techniques to tune the charge carriers in semiconductors (e.g., Li et al. stated vertical MoS2 p-n junction by AuCl3 to doped p-type and benzyl viologen (BV) as n-dopant to make an ultimate thin MoS2 p-n junction diode).107 However, in such chemical doped devices, the rectifcation ratio is limited.107 That’s why electrostatic doping is considered most effective technique in such van der Waals diodes.

4.5.3

HIGH-FREQUENCY DEVICES

The load transient response, bandwidth, and speed of the electronic devices could be improved through the high-frequency devices. The large bandwidth and high speed are only achievable if a device is operating at a low voltage drop and minimum power loss to provide a higher switching frequency.28,108 For this purpose, using different metal contacts the organic semiconductors, or metal oxides were used to fabricate the Schottky diodes over the silicon chips or glass.109–114 These Schottky diodes could operate up to the maximum frequency range of 900 MHz but could not be used for the fexible or wearable devices because of the lack of fexibility. Wearable and fexible electronic devices have been expected because of the suddenly growing thin flms and graphene devices, but to date, no reasonable devices have been introduced yet. J. Zhang et al., fabricated a Schottky diode based on the indium-gallium-zincoxide (IGZO) thin flm, which could operate up to a high frequency of 2.45 GHz, but the IGZO-thin flm growth over the fexible substrate is still challenging.90

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FIGURE 4.11 (a) The transfer characteristics of the monolayer WSe2 device at a fx biasing voltage of 0.2V. The Ids versus Vg cyclic-curve is extracted after sweeping the gate voltage from −20V to +20V. (b) Band diagram of the monolayer WSe2 device while operating as a p-n diode. (c) The output characteristics of the device are plotted as I-V curves. The inset is showing how to fx the metal gates. The dashed green line is for n-n at fxed VG1  = VG2  = 40 V, p-p is presented in dashed blue line for VG1  = VG2  = −40 V, the solid green line is for p-n at fxed  VG1  =  −40  V,  and VG2  =  40  V, and the solid blue line is for n-p at fxed VG1 = 40 V, VG2 = −40 V. Source: (ref.106) Copyright 2014, Nature Publishing Group

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FIGURE 4.12 The vertically stacked p-i-n diode composed of Gd/WSe2/Pt is showing a high-frequency operating limit. (a) The transmission coeffcient (|S21|) is plotted at T = 4.2 K as VSD, and driving frequency is changed. (b) High-frequency switching (|S21|) at differing frequencies. (c–d) The line profle and two-dimensional display, (c) line profles, (d) the simulated junction admittances under an active Gd/WSe2/Pt junction area of 7.068 μm2, showing that the 2D vdW vertical diodes could be used for high-frequency switching applications. Source: (ref.79) Copyright 2018, Nature Publishing Group

To overcome these challenges, 2D materials are considered the best candidates over the 3D bulk materials to fabricate the high-frequency smart devices.27,88,115,116 Recently, a fast and ultrahigh frequency switching of 10 GHz is reported using few nanometers thick sheet of tungsten di-selenide (WSe2).79 The WSe2 sheet was sandwiched between the Pt (used as a high work function metal) and Gd (used as a low work function metal) to fabricate a tunneling device, as shown in Figure 4.10 (a–b). The Gd is a low work function metal (Φ = 2.9 eV), and it doped the top surface of the WSe2 to n-type, while the high work function of Pt (Φ = 5.6 eV) generates more hole on the bottom surface of WSe2 and make it p-type. The middle layers of the WSe2 remain intrinsically insulator. This 2D TMDs-based p-i-n device can operate up to 10GHz frequency. Moreover, the frequency limits could be improved by limiting the capacitive loss through the substrate because these capacitive losses also cause to decrease the switching frequency. Such vertical diodes based on the thin sheets of the 2D TMDs semiconductor materials may provide a rout to fabricate the high-speed, fexible electronic devices. The mechanically fexible LCDs, mobiles, and wireless devices could be developed by growing a large area thin sheets of the 2D TMDs semiconductor materials.109

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Emerging 2D Materials and Applications

CHALLENGES AND OPPORTUNITIES OF POST-GRAPHENE 2D MATERIALS

Detection and preparation of a single layer are the earlier challenges in single-layer characterization. From bulk crystal, mechanical exfoliation is the most uncomplicated and effortless method for moving 2D sheets of layered van der Waals solids like graphene to any substrate. Flakes of size 10 µm are normally fabricated from exfoliation, which varies in thickness from single to multilayer. To identify single and multiple layer fakes, the preliminary methodology with the most infuential elevated throughput is optical microscopy. Moreover, to establish the thickness of the layer, the highly potent procedure with 5% of precision is AFM. However, inconsistencies emerged from the difference in the associations of the tip with the sample and substrate. For instance, estimating the height of the second layer on a frst layer is the better determination of the solitary layer thickness, as opposed to the tallness between a solitary layer and the substrate because in the previous case the tip layer interaction is consistent. To exclude any hysteretic artifacts, the optimization of a tip surface distance is necessary to attain a highly precise height profle of a single layer. The astonishing applications of elemental 2D substances occur in the feld of electronics and sensing, which allow superb technological development in future devices. The valuable properties that exhibit in 2D materials include high sensitivity to adsorbed molecule, high on/off ratio and high electron-hole mobility, material fexibility, and insulating attitude. For several gases, B, N, and BN-doped stanine117 work as a sensor. Moreover, 2D materials having buckled or puckered structure direct the path to the anisotropy in the optical conductivity because new electronics and optoelectronics designing of gadgets are made possible. Plasmonic, ultrafast laser, and photodetectors are the major areas of electronics and optoelectronics, where amazing applications lie. Also, the technical revolution in the power and energy sector depends on such strategies that are highly robust and reliable. 2D materials play an important role in this direction, providing next-generation devices for energy storage that provide lightweight energy storage gadgets without sacrifcing performance. 2D materials also upgrade batteries and supercapacitors to offer various benefts in the formation of new and novel thermoelectric devices. Hi-tech uprising worked around basic 2D materials will require a lot of exploration, designing, also, improvement in the coming years. In the frst place, utilization for basic 2D materials should be better perceived at a fundamental level and developed to guarantee reliability during production at a large scale. Future working procedures should be customized to the application of interest with lower cost and more versatile strategies for applications, like plasmonic, photodetectors, and spintronics. Moreover, researchers must focus on the properties of 2D materials that are predicted but neglected, such as ferroelectricity, because it might be vital to opening another range of future applications. Therefore, it can be concluded that despite the huge diffculties ahead, there is no uncertainty that the outstanding properties of these exciting materials will impact the future innovation space in applications, for example, nanoelectronics, spintronics, detecting and sensing, thermoelectric, photonics, and energy frameworks.71,79,88,115,116

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82. Gan, X. et al. 2H/1T phase transition of multilayer MoS2 by electrochemical incorporation of S vacancies. ACS Applied Energy Materials 1, 4754–4765, http://doi.org/10.1021/ acsaem.8b00875 (2018). 83. Island, J. O., Steele, G. A., Zant, H. S. J. V. d. & Castellanos-Gomez, A. Environmental instability of few-layer black phosphorus. 2D Materials 2, 011002, http://doi. org/10.1088/2053-1583/2/1/011002 (2015). 84. Xu, Y., Shi, Z., Shi, X., Zhang, K. & Zhang, H. Recent progress in black phosphorus and black-phosphorus-analogue materials: Properties, synthesis and applications. Nanoscale 11, 14491–14527, http://doi.org/10.1039/C9NR04348A (2019). 85. Miao, J., Zhang, L. & Wang, C. Black phosphorus electronic and optoelectronic devices. 2D Materials 6, 032003, http://doi.org/10.1088/2053-1583/ab1ebd (2019). 86. Lince, J. R., Carré, D. J. & Fleischauer, P. D. Schottky-barrier formation on a covalent semiconductor without Fermi-level pinning: The metal-MoS 2 (0001) interface. Physical Review B 36, 1647 (1987). 87. Walia, S. et al. Characterization of metal contacts for two-dimensional MoS2 nanofakes. Applied Physics Letters 103, 232105 (2013). 88. Dastgeer, G., Abbas, H., Kim, D. Y., Eom, J. & Choi, C. Synaptic characteristics of an ultrathin hexagonal boron nitride (h-BN) diffusive memristor. Physica Status Solidi (RRL)—Rapid Research Letters n/a, 2000473, https://doi.org/10.1002/pssr.202000473. 89. Liu, Y. et al. Van der Waals heterostructures and devices. Nature Reviews Materials 1, 16042, http://doi.org/10.1038/natrevmats.2016.42 (2016). 90. Zhang, J. et al. Flexible indium-gallium-zinc-oxide Schottky diode operating beyond 2.45 GHz. Nature Communications 6, 7561, http://doi.org/10.1038/ncomms8561 (2015). 91. Jeon, P. J. et al. Black phosphorus–zinc oxide nanomaterial heterojunction for p-n diode and junction feld-effect transistor. Nano Letters 16, 1293–1298, http://doi.org/10.1021/ acs.nanolett.5b04664 (2016). 92. Sokolov, P. M. et al. Graphene-quantum dot hybrid nanostructures with controlled optical and photoelectric properties for solar cell applications. Russian Chemical Reviews 88, 370–386, http://doi.org/10.1070/rcr4859 (2019). 93. Afzal, A. M., Dastgeer, G., Iqbal, M. Z., Gautam, P. & Faisal, M. M. High-performance p-BP/n-PdSe2 near-infrared photodiodes with a fast and gate-tunable photoresponse. ACS Applied Materials & Interfaces 12, 19625–19634, http://doi.org/10.1021/ acsami.9b22898 (2020). 94. Fan, X. F., Shen, Z. X., Liu, A. Q. & Kuo, J. L. Band gap opening of graphene by doping small boron nitride domains. Nanoscale 4, 2157–2165, http://doi.org/10.1039/ c2nr11728b (2012). 95. Muchharla, B. et al. Tunable electronics in large-area atomic layers of boron-nitrogencarbon. Nano Letters 13, 3476–3481, http://doi.org/10.1021/nl400721y (2013). 96. Fiori, G., Betti, A., Bruzzone, S. & Iannaccone, G. Lateral graphene-hBCN heterostructures as a platform for fully two-dimensional transistors. Acs Nano 6, 2642–2648, http://doi.org/10.1021/nn300019b (2012). 97. Miao, J. et al. Vertically stacked and self-encapsulated van der Waals heterojunction diodes using two-dimensional layered semiconductors. ACS Nano 11, 10472–10479, http://doi.org/10.1021/acsnano.7b05755 (2017). 98. Sachs, B., Wehling, T. O., Katsnelson, M. I. & Lichtenstein, A. I. Adhesion and electronic structure of graphene on hexagonal boron nitride substrates. Physical Review B 84, http://doi.org/10.1103/Physrevb.84.195414 (2011). 99. Gopalan, D. P. et al. Formation of hexagonal boron nitride on graphene-covered copper surfaces. Journal of Materials Research 31, 945–958, http://doi.org/10.1557/jmr.2016.82 (2016).

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116. Dastgeer, G., Shehzad, M. A. & Eom, J. Distinct detection of thermally induced spin voltage in Pt/WS2/Ni81Fe19 by the inverse spin hall effect. ACS Applied Materials & Interfaces 11, 48533–48539, http://doi.org/10.1021/acsami.9b16476 (2019). 117. Molle, A. et al. Buckled two-dimensional Xene sheets. Nature Materials 16, 163–169, http://doi.org/10.1038/nmat4802 (2017).

5

2D Material Photonics and Optoelectronics Muhammad Farooq Khan, H. M. Waseem Khalil, Shania Rehman, Muhammad Asghar Khan, and Jonghwa Eom

CONTENTS 5.1

Broadband Photodetectors ...............................................................................71 5.1.1 Photovoltaic Devices Based on 2D Materials......................................73 5.2 2D Optoelectronic Memory Devices ...............................................................75 5.2.1 ORAMs Manifested by 2D Materials ................................................ 77 5.2.2 ORAMs Manifested by 2D Materials’ Heterostructures ....................78 5.3 Bio-Photonics of 2D Materials ....................................................................... 80 5.4 Light-Emitting Diodes (LEDs) of 2D Materials............................................. 83 5.5 Photodetectors of MXene ............................................................................... 87 References ................................................................................................................ 89

5.1 BROADBAND PHOTODETECTORS Broadband photodetectors have a vital role in the new era technology of medical sciences, wireless communication, telecom engineering, etc. The broadband light having wavelengths (λ) larger than the energy bandgap (eV) of material will work for any photodetector. To detect light, the photoelectric effect or photothermal effect is often employed for successful experiments. Photodetector follows the mechanism of photoelectric effect, where an electron is ejected from the valance band by an incident photon of a light source to the conduction band. Recently 2D materials such as graphene TMDCs achieved gigantic consideration from researchers in optoelectronics devices because of their impressive optical and remarkable electrical properties [1–7]. Usually, thin-flm semiconductors for broadband light detection have narrow bandgaps of   1,000 cycles), and stable retention (>  104  s). At 10 ms gate pulses of +10 V and −10 V, a successive program and erase states are observed, respectively.

5.3 BIO-PHOTONICS OF 2D MATERIALS The growing demands of diagnostics diseases and biomedical applications have signifcantly encouraged the rational structure and synthesis of an inclusive range of functional nanomaterials. The association of 2D nanomaterials with the recognition of biomolecular events establishes the sensing and diagnostic investigations.

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2D materials are among the most studied materials in the twenty-frst century of nanotechnology feld. Materials such as graphene and TMCs attracted a plethora of attention because of their wide range of properties leading to numerous applications in biosensing and bio-photonics [46–50]. Because of their exceptional optical properties, 2D materials will emerge in biomedical photonic applications once their bio-compatibility is satisfactory, depending upon their chemical and physical modifcations. We address the latest advancement in bio-photonics of 2D materials and their imaging applications in this section. A biosensor is a diagnostic device that is used to locate biochemical substances and the concentration or existence of different kinds of biological analytes, like antibodies, DNA, enzymes, proteins, or antigens. There are three components of a biosensor [51]. 1. The probe 2. The transducer component 3. The electronic system The function of the probe is to identify the analyte that is of main concern. The transducer transforms the event of bio-recognition into a quantifable signal, and the electronic system that reads the output is often used in conjunction with the transducer [52]. Mainly, optical biosensors of 2D materials are focused. In 2D materials, it is widely known that a nanosheet is exceedingly susceptible to the differences on its surface. For example, an optical biosensor based on graphene exhibited single-cell fow detection with excellent resolution and sensitivity [53]. Down to single cells, this sensor can differentiate between tiny normal lymphocyte cells and bigger malignant, cancerous cells. Furthermore, the dynamic range is wide and easily adjustable by modifying the angle of incidence and power of incidence. Recently, MoS2 has been utilized in a photo-electrochemical sensor to detect miRNA. The photoanode comprises Au NPs and MoS2 NSs heterojunction on ITO [51]. The photoresponsive material is the MoS2-Au NPs heterojunction, which improves the visible light-harvesting effciency of MoS2. The MoS2 sensor on ITO with Au NPs displays high sensitivity for miRNA detection, according to photocurrent measurements, with the capacity to differentiate interrelated and single-base mismatched miRNA. For a moment, the graphene-based optical refractive index sensor showed effectively good resolution (1.7 × 10 −8) and enhanced sensitivity of (4.3 × 107) mV/RIU with extensive dynamic range, see Figure 5.8 [54]. This sensor can precisely detect a minor amount of cancer cells among other normal cells at the single-cell level, which provides a true statistical distribution of cancer and normal cells by fewer cells. Further, it is a matter of great importance to detect specifc DNA or peptide sequences for several biomedical applications, including disease diagnostics, biomolecular analysis, gene therapy, environmental forecast, etc. In 2014, Loan and the group designed a device structure of graphene/MoS2 to use for selective and label-free detection of DNA hybridization, see Figure 5.9 (a–b) [50]. Recently, Shorie et al. made a nanohybrid mediated SERS substrate, which was established by incorporating the Au NPs on exfoliated NSs of WS2 to make plasmonic concentrations. High selectivity for cardiac marker myoglobin was achieved by functionalizing the

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FIGURE 5.8 (a) Flow-sensing system set up for a single cell. (b) Micrographs of Jurkat cells and lymphocytes with scale bar of 15 μm (left and right side, respectively). (c) Distinct timedependent variation in voltage linked with mixed lymphocytes and Jurkat cells. Source: Reprinted with the permission of [54], copyright from American Chemical Society

FIGURE 5.9 (a) Schematic drawing of DNA detection technique by microscope and structure of sensor composed by graphene/MoS2 heterostructure along with (b) the photoluminescence (PL) peak area mappings. (c) Schematic representation of SERS device. (d) Calibration curve of Mb (Myoglobin) with related Raman intensity. (e) Schematic structure of as-prepared NIR phototransistor. Source: Reused with the permission of (a) and (b) from [50], copyrights from Wiley, (c) and (d) [55], copyrights from Springer, and (e) [56], copyrights from Elsevier

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surface of the nanohybrid with specifc aptamers, Figure 5.9 (c, d) [55]. This investigation demonstrates a paragon to synergistically manipulate the distinct electromagnetic and chemical characteristics of Au NPs and WS2 for many-fold augmentation of SERS signals. In addition, Zhai et al. designed an up-conversion optogeneticsinspired NIR phototransistor synapse, manifested by up-converting nanoparticles sensitized MoS2 [56]. In this device, MoS2 could be used as light-sensitive ion channels to reabsorb the visible light emitted from up-converting nanoparticles under near-IR illumination, see Figure 5.9 (e). This idea of bringing biology, optics, and electronics close together can open new routes for emerging optogenetics-inspired neuromorphic technology in bio-photonics.

5.4

LIGHT-EMITTING DIODES (LEDs) OF 2D MATERIALS

The LEDs are considered to be the most valuable optoelectronic innovation in the past few decades, even winning the 2014 race for the Nobel Prize in Physics. LED is a semiconductor device that emits light (visible or invisible) when energized. In detail, when a p-n junction is forward biased, recombination of charges (electrons and holes) takes place within the structure and mostly near to the p-n junction. This recombination process requires that the energy possessed by the free electron be released or transferred in the form of heat or in the form of photons. The color or wavelength of the released light depends on the bandgap of the semiconducting material used for the making of LEDs. LEDs have several advantages over an incandescent light source, including brighter light, longer lifetime, less combustion, and smaller size. Various types of LEDs have been studied and commercialized in recent years. The two-dimensional materials have appealed to great attention in recent times due to their captivating optical, mechanical, and electrical properties, which are originated from their unique band structure and morphology [12, 57–59]. Graphene, a monolayer of carbon atoms hexagonally packed like a 2D honeycomb lattice structure, has excellent carrier movement and broadband light absorption [57, 60]. Graphene has a commendably modifable charge carrier density, which makes it a promising contender for electrically tunable devices. However, with the lack of bandgap in graphene, 2D TMDs (MoS2, WS2, MoSe2, MoTe2, WSe2, etc.) have become promising layered semiconductors, having a bandgap in the range of 1.57 to 2eV [61], which is more pertinent for making LEDs. Besides the layered materials, there is PbS, a non-layered 2D material having a direct bandgap of around 0.4 eV, which has great potential in a broad range of wavelength detection from visible region to IR [62, 63]. The ultrahigh carrier mobility, a wide range of light emission, and powerful light-matter interaction of 2D materials make them suitable contenders in optoelectronics, such as LEDs and photodetectors. It is important to fnd single-layer TMDs with direct bandgap structure and large light-emitting profciency. The properties of light emission in 2D TMDs depend on the excitation effect of the materials [14, 64]. Strong Coulombic force interaction and charged carriers generated electrically or optically in TMDs remarkably give rise to a multiple-particle phenomena such as the generation of a different kind of exciton (e-h pair), including trion (two h+s with an e⁻ or vice versa) and bi-exciton (two h+s with two e⁻s) [65]. In these formations, trion exhibits a charged feature, whereas

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FIGURE 5.10 (a) Schematic of single-layer WSe2 p-n junction devices. (b) Optical image of the multiple monolayer WSe2 p-n junction devices. The scale bar is 4 μm. (c) The variation of current values. (d) The electroluminescence image (red) placed over the device image (grayscale). (e) Band diagram and device schematic drawing the electroluminescence generation from valley excitons. Source: Reproduced with the permission of [74], copyrights from Nature

exciton and bi-exciton express a charge-neutral feature [66]. In 2D materials, there are several ways to create p-n junctions for LEDs, such as chemical doing, electrostatic gating, van der Waals heterostructures, and single photons emitters [67–71]. The most common technique used to build LEDs is electrostatic gating, which uses two local gates to create a p-n junction within the 2D material sheets [72, 73]. Since monolayer WSe2 is one of the promising TMD materials, it has been widely used to create LEDs based on the p-n junction with a large photon emission rate (~16 million s−1) at an applied current of 35 nA, as shown in Figure 5.10 [74]. Despite all that mentioned in the preceding section, the previous study reveals that ambipolar transport behavior can be encouraged in a feld-effect transistor with conventional gate dielectrics that may have serious issues of dielectric breakdown [73, 75]. To overcome the dielectric breakdown problem, an ionic liquid gated structure

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FIGURE 5.11 The optical microscope images of fnal devices after Au contacts for (a) mono- (ML) and bilayer (BL) and (b) only ML fakes. (c) Schematic drawing of the ionic liquid gated FET of monolayer WS2 devices in unipolar injection regime and (d) bipolar injection regime. (e) The optical image of the monolayer WS2 device with visible emitted light spot due to electroluminescence. (f) A comparison of the normalized electro- and photoluminescence spectra of monolayer WS2. Source: Reprinted with the permission of [76], copyrights from the American Chemical Society

is used, which demonstrates unipolar and bipolar injection regimes, see Figure 5.11 [76]. In this work, Jo et al. designed an LED of a monolayer WS2 with ionic liquid gated FET. Under bias conditions, this device shows an emitted light spot because of electroluminance with high visibility. The position of the emission spot in an ambipolar

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regime can be easily shifted by changing the gate and source-drain voltage. The effective electrons and holes are accumulated in the FET, execute the operation in an ambipolar injection regime, with electrons and holes injected at the opposite electrodes of the device simultaneously, which leads to light emission. The electroluminescence and photoluminescence spectra for the monolayer device are similar to each other, as expected [76]. Notably, van der Waals heterostructures of 2D semiconductors are the backbone of solid state physics and allows a variety of 2D materials with different crystal axes orientations and lattice constant to be combined [77]. Recently, Ye et al. studied the microscopic investigation of the electroluminescence from a diode manifested by the assembly of heavily p-type doped silicon and monolayer MoS2 [78]. Therefore, to check the device performance, a voltage is applied to the MoS2/silicon heterojunction, as shown in Figure 5.12. In this work, the injection of holes from silicon across the junction gives rise to a recombination process due to the direct bandgap of monolayer MoS2. The electroluminescence spectrum of the device is found at room and low temperatures under different current values. The electroluminescence spectra exhibit a current threshold of about 15 μA; however, the basic features of the excitation emission (AX and DX) are observed at currents higher than threshold. Similarly, in 2017 Ross et al. designed a lateral p-n junctions of WSe2 and MoSe2 hetero-bilayer, which electrostatically determined the interlayer exciton optoelectronics, see Figure 5.13 [79]. Since the application of a forward bias facilitates the early detection of electroluminescence from interlayer excitons. The p-n junction at the neutral bias performs as the most sensitive photodetector, where the wavelengthdependent photocurrent measurement permits the direct observation of resonant optical excitation of the interlayer exciton.

FIGURE 5.12 (a) Diagrammatic representation of the monolayer MoS2/Si heterojunction. (b) Model band structure of the MoS2 diode under a forward bias voltage. (c) Electrical characteristics of the monolayer MoS2 diode. Inset: surface plot of the electroluminescent emission. (d) At room temperature and (e) at low temperature, the electroluminescence (EL) spectra of a MoS2 diode were recorded at different current magnitudes. Source: Reproduced with the permission of [78], copyrights from the American Institute of Physics

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FIGURE 5.13 (a) Top view of a microscope image of the device. The scale bar is 2 μm. (b) Drawing of device region (HS2) cross-section. (c) Micrograph and electroluminescence image of the device area (top and bottom, respectively). Scale bars are of 2 μm (d) spectra of EL (red) and photoluminescence (black) for the HS2 region. Source: Reprinted with the permission of [79], copyrights from the American Chemical Society

The interlayer exciton has about 200 times lower photocurrent amplitude as compared to that of the resonant excitation. This shows that the spatial separation of holes and electrons affects the oscillator strength of the interlayer exciton to a level of two orders of magnitude.

5.5 PHOTODETECTORS OF MXene Based on their phenomenal physical and chemical characteristics, MXenes have gained much attraction as a group of 2D layered materials [80, 81]. The general chemical formula of MXene is M n+1X n Tx , where n = 1, 2, or 3. M represents transition metal (Ta, Mo, Cr, Hg, Zr, and Ti); X denotes carbon or nitrogen; and Tx is a surface functional group (hydroxyl, oxygen, chlorine, or fuorine) [82, 83]. Several types of MXenes have been obtained by experimentation, and a lot more have been predicted theoretically [84–86]. Owing to their stochiometric properties and various surface functionalization, MXenes have shown tunable properties. In particular, Ti3C2Tx (type of MXene) has been verifed to be a high optical transmittance (~93%) and conductivity (~5736 cm−1) when its thickness is ~4 nm [87, 88]. Another exciting feature of MXenes is the tunable work function (~2.14 eV to ~5.65 eV) that covers a wide range of materials [89]. Meanwhile, unique optical and electrical properties can be fnely tuned by transition metals, surface termination, thickness, electrochemical behavior, etc. [90]. Subsequently, these tunable optoelectronic properties make MXenes a promising candidate for many future applications, like transparent conductors, photodetectors, light-emitting diodes, photovoltaic devices, energy storage devices [85, 91–93]. However, a photodetector is considered to be an essential optoelectronic device and can convert incident light into current or voltage, having a wide range of applications, such as image sensing, night visions, and communications. Despite their rich optical and electrical properties, 2D MXenes photodetectors are not well explored in the research community. However, the recent development in 2D MXenes based

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FIGURE 5.14 (a) Schematic of design fexible photodetectors, parallel-type thin-flm Mo2CTx. (b) Zoomed-in view of one device from an array. (c) Photo-response of a Mo2CTx photodetector at 0.7 V bias. Inset: Photograph of the photocurrent arrangement. (d) I-V characteristics of the Mo2CTx photodetector in the dark and under different light intensities of a wavelength of 660 nm. (e) The ratios of the photocurrent to the dark current of the Mo2CTx thin-flm photodetector versus different light intensities. (f) Photo-response of Mo2CTx photodetector, the light-switch ON and OFF cycles were ~500 ms at a wavelength of 660 nm. Source: Reprinted with the permission of [94], copyrights from the Wiley

photodetectors over the past few years cannot be ignored. There are several reports on the MXenes based photodetectors, including simple photodetector, plasmonenhanced photodetector, and self-driven photodetector [94, 95][16]. Photodetectors based on fve different MXenes nanosheets (Mo2CTx, T2CTx, T2CTx, Ti3C2Tx, Ti3C2Tx, Nb2CTx, and Nb2CTx) are fabricated, among which Mo2CTx shows promising results [94]. In 2019, Velusamy et al. deposited thin flms on paper substrates, which demonstrated high-level responsivity and detectivity (9 A W−1 and ~5 × 1011 Jones, respectively) in a broad range of 400–800 nm with consistent photoswitching characteristics at λ  =  660 nm. Moreover, Mo2CTx thin-flm devices are found to be resistive to change in the natural environment along with a prolonged exposure to light and mechanical forces, making them useful in the visible range of light spectrum, see Figure 5.14. Recently, Deng et al. demonstrated a photodetector of combined 2D material and MXene. This photodetector is manifested by an all-sprayed-processable method having a large area on common paper by 2D CsPbBr3 and conductive Ti3C2Tx (MXene) [96]. This is a fexible photodetector and successfully revealed the photo communication with an on/off current ratio up to 2.3 × 103 and an outstandingly fast response to light exposure (~18 ms), see Figure 5.15. Also, after bending the 1,500 cycles,

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FIGURE 5.15 (a) Diagrammatic representation of a perovskites/MXenes photodetector under illumination. (b) The SEM image of Ti3C2Tx. (c) The SEM image of CsPbBr3. (d) Timeresolved photocurrent of the photodetector at a different thickness of MXene electrodes. (e) Time-resolved photocurrent of the photodetector arrays at several bending angles from 180° to 60. (f) Diagrammatic representation of energy band structure of the perovskites/MXenes photodetector at the equilibrium state (left) and under light (right). Source: Reprinted with the permission of [96], copyrights from the Wiley

the devices still preserve exceptional fexibility and stability. This work offers an economical process of enormously creating fexible large-area photodetectors for extended photo communication and wearable technology.

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87. Hantanasirisakul K, Zhao M Q, Urbankowski P, Halim J, Anasori B, Kota S, Ren C E, Barsoum M W and Gogotsi Y 2016 Fabrication of Ti3C2Tx MXene transparent thin flms with tunable optoelectronic properties Adv. Electron. Mater. 2 1600050 88. Zhang C, Anasori B, Seral‐Ascaso A, Park S H, McEvoy N, Shmeliov A, Duesberg G S, Coleman J N, Gogotsi Y and Nicolosi V 2017 Transparent, fexible, and conductive 2D titanium carbide (MXene) flms with high volumetric capacitance Adv. Mater. 29 1702678 89. Kang Z, Ma Y, Tan X, Zhu M, Zheng Z, Liu N, Li L, Zou Z, Jiang X and Zhai T 2017 MXene-silicon Van Der Waals heterostructures for high‐speed self‐driven photodetectors Adv Electron. Mater. 3 1700165 90. Zhang X, Shao J, Yan C, Qin R, Lu Z, Geng H, Xu T and Ju L 2021 A review on optoelectronic device applications of 2D transition metal carbides and nitrides Mater. Des. 109452 91. Dillon A D, Ghidiu M J, Krick A L, Griggs J, May S J, Gogotsi Y, Barsoum M W and Fafarman A T 2016 Highly conductive optical quality solution‐processed flms of 2D titanium carbide Adv. Funct. Mater. 26 4162–4168 92. Ahn S, Han T H, Maleski K, Song J, Kim Y H, Park M H, Zhou H, Yoo S, Gogotsi Y and Lee T W 2020 A 2D titanium carbide MXene fexible electrode for high‐effciency light‐emitting diodes Adv. Mater. 32 2000919 93. Chaudhari N K, Jin H, Kim B, San Baek D, Joo S H and Lee K 2017 MXene: An emerging two-dimensional material for future energy conversion and storage applications J. Mater. Chem. A 5 24564–24579 94. Velusamy D B, El‐Demellawi J K, El‐Zohry A M, Giugni A, Lopatin S, Hedhili M N, Mansour A E, Fabrizio E D, Mohammed O F and Alshareef H N 2019 MXenes for plasmonic photodetection Adv. Mater. 31 1807658 95. Zhang X, Shao J, Yan C, Wang X, Wang Y, Lu Z, Qin R, Huang X, Tian J and Zeng L 2021 High performance broadband self-driven photodetector based on MXene (Ti3C2Tx)/ GaAs Schottky junction Mater. Des. 109850 96. Deng W, Huang H, Jin H, Li W, Chu X, Xiong D, Yan W, Chun F, Xie M and Luo C 2019 All‐sprayed‐processable, large‐area, and fexible perovskite/MXene‐based photodetector arrays for photocommunication Adv. Opt. Mater. 7 1801521

6

Magnetic Properties of 2D Materials Rajesh Katoch and Shilpee Jain

CONTENTS 6.1 6.2

Introduction .................................................................................................... 95 2D Magnetic Materials and Properties........................................................... 96 6.2.1 Graphene............................................................................................. 96 6.2.2 Transition Metal Dichalcogenides (TMDs) ........................................ 97 6.2.3 Transition Metal Halides (TMHs) ...................................................... 98 6.2.4 Ternary Transition Metal Chalcogenides and Halides (TMCHs)......101 6.2.5 Transition Metal Phosphorous Trichalcogenides (TMPCs)...............103 6.2.6 Topological Insulators........................................................................105 6.3 Synthesis and Characterization of 2D Materials...........................................107 6.3.1 Top-Down Approach .........................................................................107 6.3.2 Bottom-Up Approach ........................................................................108 6.3.2.1 Chemical Vapor Deposition (CVD) ....................................108 6.3.2.2 Molecular Beam Epitaxy (MBE)........................................108 6.3.2.3 Wet Chemical Methods.......................................................109 6.3.3 Characterization Techniques .............................................................109 6.3.3.1 Vibrating-Sample Magnetometer (VSM) ...........................109 6.3.3.2 Superconducting Quantum Interference Device Magnetometer (SQUID)......................................................110 6.3.3.3 Magneto-Optic Kerr Effect (MOKE) .................................110 6.3.3.4 Magnetic Force Microscopy ...............................................110 6.4 Applications of 2D Magnetic Materials ........................................................ 111 6.4.1 Spin Valve.......................................................................................... 111 6.4.2 Hall Effect Sensors ............................................................................ 111 6.4.3 Spin Field-Effect Transistors ............................................................. 111 6.4.4 Magnetic Tunnel Junction (MTJ) ......................................................112 6.5 Conclusions and Outlook...............................................................................112 References...............................................................................................................113

6.1

INTRODUCTION

Magnetism is a quantum mechanical phenomenon that arises due to alignment of neighboring spins of individual atoms over a characteristic length scale, driven by exchange interactions between the neighboring spins. The characteristic length scale over which this alignment extends is governed by the strength of the exchange interactions, magnetostatic energy, magnetic anisotropy, etc. At fnite temperatures, DOI: 10.1201/9781003247890-6

95

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reduction in dimensions brings an abrupt end to the long-range order and weakening of magnetic interactions. In thin flms, the shape anisotropy tends to align magnetic moments in-plane to minimize the demagnetization feld. However, at the surface of magnetic flms, surface anisotropy causes the spins alignment perpendicular to the surface while the exchange energy aligns the spins parallel to each other. This causes a large demagnetization feld, which destroys the magnetic ordering and increases the coercivity of the flms. For bulk samples, magnetostatic energy prevails over surface anisotropy and aligns the spin in-plane of the sample, thereby reducing the demagnetization feld. Likewise, magnetic ordering may be lost due to thermal fuctuations if the thermal energy is suffcient to overcome the magnetostatic energy. For instance, in small particles, as the particle diameter decreases, the anisotropy energy decreases. Consequently, below a critical size, the thermal energy can overcome the anisotropy energy and reverse the directions of spontaneous magnetization even without the application of external feld, a phenomenon called super-paramagnetism. In fact, for an isotropic two-dimensional Heisenberg system with fnite-range interactions, long-range magnetic order does not exist at fnite temperature, according to Mermin-Wagner theorem.1 However, the experimental observation of long-range order even in magnetic monolayers has renewed the dormant interest and opened up avenues of opportunities in 2D magnetic materials.2,3 It has been suggested that large magnetic anisotropy and/or long-range dipole-dipole interactions can suppress fnite temperature fuctuations and stabilize magnetic ordering in 2D materials.3 Additionally, it was suggested that the essential condition for long-range ferromagnetic ordering in 2D materials is the presence of spin-wave excitation gap, which in turn is a direct consequence of magnetic anisotropy. Due to the immense interest, the number of 2D magnetic materials are increasing by the day. In this chapter, we will focus on various 2D magnetic materials and their properties, methods of synthesis and characterization, and their applications.

6.2 6.2.1

2D MAGNETIC MATERIALS AND PROPERTIES GRAPHENE

Any discussion on 2D material properties should ideally begin with the mention of graphene,4 which acted as a catalyst for 2D materials research due to its exceptional electrical,5 mechanical,6 optical,7 thermal properties8 and unique band structure. Graphene is identifed to be an important material for spin transport applications due to long spin diffusion length.9 In its purest form, graphene, like other graphitic allotropes, is nonmagnetic. However, the carbon atoms in the sheet are sp2 hybridized, and each sheet has an unsaturated bond, making it plausible to functionalize the pristine structure by external means to induce magnetism, which has been confrmed by several theoretical and experimental studies.9–17 For instance, a magnetic moment of 1µB per hydrogen chemisorbed defect and 1.12–1.53 µB per vacancy defect was deduced from frst principles calculations, for a single carbon atom point defect in the hexagonal lattice.10 Further, it was determined that the magnetic coupling was either ferromagnetic or antiferromagnetic depending on the defects belonging to the same or different hexagonal sublattice on graphene sheet, respectively. Another theoretical study on

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97

FIGURE 6.1 Relaxed geometric structure and electronic structure of half-hydrogenated graphene (a), half-fuorinated and half-hydrogenated graphene (b), fully fuorinated graphene (c). The arrows and numbers in (a) are for magnetic moments. Source: Reprinted with permission from Ref.13

hydrogenated graphene (graphane)13 found that the application of electric feld converts it into half hydrogenated state, which is a semiconductor with a small indirect bandgap (0.43 eV) along with the appearance of magnetic moment (0.922 µB) at the site of unsaturated C atom (Figure 6.1(a)13), which polarizes its neighboring C(−0.141 µB) and H(+0.219 µB) atoms. The unsaturated C atoms couple ferromagnetically, mediated by the hydrogenated C atom via p-p interactions. Further decorating the half-hydrogenated sheet with F atoms (Figure 6.1 (b)) yields a nonmagnetic semiconductor with direct bandgap (3.7 eV), while a fully fuorinated graphene structure remained nonmagnetic, direct bandgap (3.4 eV) semiconductor (Figure 6.1 (c)). Similar studies reveal that functionalizing graphene with F-atoms creates an antiferromagnetic semiconductor that stabilized to ferromagnetic metallic state on application of magnetic feld with a resultant large room temperature magnetoresistance (2,200%).14 The magnetic properties of graphene quantum dots (GQDs) of various shapes depend on the edge states: the zero-energy states (ZESs) give rise to spin paramagnetism, whereas the dispersed edge states (DESs) are responsible for a temperature independent diamagnetic character. Hexagonal, circular, and randomly shaped GQDs are diamagnetic as they contain mainly DESs. The edge states of the triangular GQDs are of ZES type, giving them a paramagnetic character.15 In a recent study, simultaneous application of strain, temperature, and magnetic feld resulted in inducing magnetism in liquid phase exfoliated graphene fakes and observation of magnetic domains by magnetic force microscopy (MFM).18

6.2.2

TRANSITION METAL DICHALCOGENIDES (TMDS)

MoS2 is the most stable layered TMD in nature. It consists of three covalently bonded hexagonal atomic layers (i.e., S-Mo-S), with middle layer of Mo atoms sandwiched between two layers of S atoms with the interlayer separation ~ 6.2 Å, due to which the van der Waals forces between the adjacent layers are very weak. Consequently,

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individual layers can be easily exfoliated by chemical or mechanical means. Its tunable bandgap (i.e., Eg ~1.3 eV (indirect) in bulk to Eg ~1.8 eV (direct) in isolated layers),19 large in-plane mobility (200 cm2 V−1s−1 ),20 and mechanical stability21 make it one of the most widely investigated TMDs. Magnetism in MoS2 has been extensively investigated theoretically and experimentally.22 SQUID magnetometry measurements on single crystals of MoS2 (~10–100 µm thick) displayed weak ferromagnetism in otherwise diamagnetic system, and the ferromagnetism persisted from 10 K up to RT. The existence of ferromagnetism was attributed to the presence of zigzag edges at the grain boundaries.22 Due to the sandwich structure of MoS2, a 2D diluted magnetic semiconductor (DMS) like state has been predicted for MoS2 from frst principles calculations by doping with transition metal ions at Mo site.23 From binding energy considerations, ferromagnetism was observed to be stable in MoS2 monolayer for Mn, Zn, Cd, and Hg doping; antiferromagnetic for Fe and Co doping; whereas any magnetic ordering is suppressed by Jahn-Teller distortions in other dopants of these groups. These observations were further supported by a subsequent study on Mn doped monolayer MoS225, which showed that dopant concentrations in the range of 10–15% might be suffcient to provide ferromagnetism in Mn/MoS2 DMSs at room temperature. To validate these fndings, in situ doping of monolayer MoS2 with Mn via vapor phase deposition techniques was investigated.26 Photoluminescence (PL) and X-ray photoelectron spectroscopy (XPS) revealed substrate dependence of modifcations to the band structure by incorporation of Mn. It was revealed that only inert substrates (i.e., graphene) permit the incorporation of signifcant amount of Mn (2%) in MoS2, while with reactive substrates (i.e., SiO2 and sapphire), incorporation of Mn was not successful. Theoretical studies predict giant spin-splitting induced by spinorbit coupling in MoS2 monolayers, which results in an out-of-plane spin polarization.27 Based on this, atomically thin MoS2 based spin-valves (i.e., NiFe/ MoS2 /NiFe) were fabricated.24 The junctions show spin-valve signals up to 240 K with moderate MR of ∼0.4% at 10 K (Figure 6.224). The MR dramatically improved to 0.73% when the NiFe(Py) layer was capped with a thin gold layer. A recent study has demonstrated room temperature ferromagnetism in in-situ Fe-doped MoS2 monolayers grown by chemical vapor deposition (CVD) technique.28 Fe-doped MoS2 related emission in magnetic circular dichroism (MCD) and PL studies showed circular dichroism (CD) (ρ ≈ 40%) at both 4 K and RT. Since luminescence in transition metal loses its CD above TC, the presence of it at 300 K is a signature of ferromagnetic state, which is further verifed by the presence of magnetic hysteresis loops at 4 K and 300 K.

6.2.3 TRANSITION METAL HALIDES (TMHS) CrI3: Bulk magnetic susceptibility and magnetization measurements show that CrI3 is a soft ferromagnet below the Curie temperature (Tc = 61 K), with its easy axis pointing perpendicular to the layers, and a saturation magnetization consistent with a spin S = 3/2 state of the Cr atoms.29 Another magnetic phase transition at T ≈ 50 K is found in low-feld magnetization measurements with B applied parallel to the layers, but there is a lack of understanding about its nature and details of the magnetic state currently.30 Low temperature magneto-optical Kerr effect (MOKE) measurements on atomically thin CrI3 crystals showed hysteretic Kerr rotation at low felds (B ~ 0.1T),

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FIGURE 6.2 (a) Magnetoresistance (MR) curves at different temperatures. The spinvalve effect is found up to 240 K, and the TMR ratio decreases as temperature rises up. (b) Characterizations of the devices with Py/MoS2/Au/Py/Co structure. The junction resistance versus temperature, showing a metallic behavior. The linear I-V curve (inset) exhibits an Ohmic contact between MoS2 and electrodes. (c) MR curves at different temperatures. The spin valve effect is found up to 120 K, and the MR ratio increases as the temperature is reduced. Source: Reprinted with permission from Ref.24

applied perpendicular to the plane of the layer, which saturated at high felds in mono and trilayer samples only.2 In the case of bilayers, the Kerr angle vanishes at low felds and only shows non-hysteretic Kerr rotation at high felds (B ~ 0.65T). This feld dependence of Kerr angle suggests that each monolayer of CrI3 possesses fnite magnetization perpendicular to the layer that corresponds to the easy axes of magnetization in bulk crystals. Likewise, the absence of Kerr rotation in bilayers suggests that the magnetization of subsequent monolayers is directed opposite to each other so that the magnetization ceases to exist at B = 0. Only at felds suffciently large enough to reorient and point magnetic moments in the feld direction, fnite magnetization, and Kerr rotation is observed in bilayer samples. This implies that interlayer exchange coupling in CrI3 monolayers is antiferromagnetic in nature. This

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FIGURE 6.3 Large tunneling magnetoresistance in vertical junctions. (a–f) Tunneling resistance (left axis) and resistance ratio R(B)/R(2 T) (right axis) of the device measured at the temperature indicated in each panel (with V  = 0.5 V and B applied perpendicular to the CrI3 layers). The red and black dots correspond to data measured upon sweeping the feld in opposite directions as indicated by the horizontal arrows of the corresponding color. The resistance ratio increases upon lowering temperature and reaches 8,000% at 10 K. The arrows of different color point to the magnetoresistance jumps that are seen in all devices, irrespective of the thickness of the CrI3 crystal. Jump J1 is always accompanied by hysteresis; at low temperature, jumps J2 and J3 occur in all devices at the same value of the applied magnetic feld, irrespective of sweeping direction. All jumps shift to lower feld values upon increasing temperature and disappear above 50 K.

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is in stark contrast with the ferromagnetic exchange interaction in bulk CrI3 crystals at low temperature.29 However, ab initio calculations for the high temperature phase (T > 200 K)29 suggest the presence of antiferromagnetic interlayer exchange interactions.31–33 Very large tunnel magnetoresistance (TMR) ~ 10,000% depending on the magnetic state of layered CrI3 was obtained in a recent study,30 demonstrating strong coupling between transport and magnetism in magnetic van der Waals semiconductors (Figure 6.3). A recent study has shown that in bilayer CrI3, pressure induces a transition from layered antiferromagnetic to ferromagnetic phase.32 Further, using scanning magnetic circular dichroism (MCD) microscopy, it was shown that in trilayer CrI3, pressure can create coexisting domains multiple phases, i.e., ferromagnetic (one) and antiferromagnetic (two). The observed changes in magnetic order were explained by changes in the stacking arrangement.

6.2.4

TERNARY TRANSITION METAL CHALCOGENIDES AND HALIDES (TMCHS)

Cr2Ge2Te6: Another 2D vdW material system for which persistent magnetism down to monolayer is demonstrated is Cr2Ge2Te63. Bulk Cr2Ge2Te6 is a semiconducting ferromagnetic below 68 K with an out-of-plane easy axis of magnetization and small coercivity (3.4 mT).34,35 To obtain 2D vdW layers, Cr2Ge2Te6 atomic layers were mechanically exfoliated with adhesive tape on 260 nm SiO2/Si substrates.3 Based on the temperature and magnetic-feld-dependent magneto-optic Kerr microscopy, long-range ferromagnetic order in pristine Cr2Ge2Te6 bilayers was established. The Kerr rotation intensity under 0.075T fell to zero at 30 K for bilayer samples, indicating the destruction of magnetic ordering. The temperature-dependent magnetic measurements revealed strong dimensionality effect as the Tc monotonically decreased for 68 K for bulk samples to about 30 K in the bilayer samples, indicating that interlayer magnetic interactions are essential for establishing the ferromagnetic order in Cr2Ge2Te6 samples. Likewise, feld dependence of Tc in different thickness samples displayed a rapid increase in Tc with increase in feld intensity at low felds (B ≤ 0.3T), providing an effective way of engineering the magnetic anisotropy with magnetic felds. The ability to tune magnetism in 2D materials with electric feld opens up new avenues to develop compact spintronic devices.36–39 For instance, electrostatic gating of thin (~22 nm) Cr2Ge2Te6 crystals was shown to modulate the magnetic phase transition and magnetic anisotropy and increase its Curie temperature.39 The exfoliated Cr2Ge2Te6 in contact with a polymer gel forms electric double-layer transistor device with high carrier density (~4×1014 cm−2 for VG = 3.9V). Magnetoresistance (MR) curves exhibited hysteresis at 60 K, 120 K, and 180 K, indicating that ferromagnetic order could be stabilized well beyond Tc at high doping levels. DFT calculations of magnetic anisotropy energy (MAE) reveal that the magnetic easy-axis rotates from out-of-plane in pristine samples to in-plane in heavily doped Cr2Ge2Te6. To explain the rotation of magnetic easy axis, it was suggested that heavy doping promotes a double-exchange mechanism mediated by free carriers, which dominates over the superexchange mechanism of the pristine samples. While interesting in its own right, the effective charge layer and enhanced ferromagnetic state is limited to top layer only (~ 1 nm). In a recent work, it was shown that the magnetic easy-axis of few-layered Cr2Ge2Te6 can be gradually tuned from an out-of-plane direction to

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in-plane direction in a solid ion conductor (SIC) gated FET device by electric feld stimulation, with tuning depth ~10 nm.40 Additionally, using anomalous Hall effect and magnetoresistance studies, the authors reported signifcant enhancement in Tc from 65 K to 180 K. Fe3GeTe3: Unlike CrI3 and Cr2Ge2Te6, bulk Fe3GeTe2 is an itinerant vdW ferromagnet with relative high transition temperature (Tc ~220–230 K) and strong perpendicular anisotropy, whose tunable magnetic properties by changing thickness, chemical composition, external gate doping, makes it very attractive for spintronic applications.38,41–43 The presence of strong magnetocrystalline anisotropy supports the sustenance of long-range magnetic order in 2D limit.42,44 Micromechanical cleavage of this material is diffcult with conventional methods, and to increase the chances of obtaining monolayer fakes, exfoliation is performed on freshly evaporated gold41 or on Al2O3.38 For instance, exfoliated Fe3GeTe2 fakes (on SiO2/Si and Au coated SiO2/Si substrates) were shown to exhibit robust 2D ferromagnetism with strong outof-plane anisotropy down to monolayer.41 Using magneto-transport (anomalous Hall effect), MOKE and RMCD measurements, a crossover from 3D to 2D Ising ferromagnetism for thickness less than 4 nm was evidenced. The transitions temperature (Tc) sharply declined from 220 K in fve layers, down to 180 K for bilayer and further down to 130 K for a monolayer, clearly showing the effect of dimensionality on ferromagnetism in thin structures Likewise, the magnetic hysteresis loops in polar RMCD shrunk as the temperature was increased and completely vanished at 130 K for a monolayer sample, suggesting the destruction of magnetic order. Another similar study on exfoliated Fe3GeTe3 layers on Al2O3 coated SiO2/Si substrates showed that as the number of layers in exfoliated Fe3GeTe3 fakes are reduced, the anomalous Hall resistance increases,38 while the ferromagnetism persists down to a monolayer. Further temperature-dependent magnetic studies revealed that at T = 100 K, ferromagnetism disappears in mono and bilayer samples, whereas it survives in thicker samples. In wafer-scale 2D ferromagnetic Fe3GeTe2 thin flms grown via MBE, the Curie temperature can be enhanced by raising the Fe composition. Additionally, upon interface coupling with antiferromagnetic MnTe, the coercive feld dramatically increases 50% from 0.65 to 0.94 Tesla upon cooling. The large-scale layer-by-layer growth and controllable magnetic properties make Fe3GeTe2 a promising candidate for spintronic applications (Figure 6.442). To further verify this strong dimensionality effect on ferromagnetism in Fe3GeTe3 system, analysis of thickness dependence of TC revealed a monotonic decrease from 180 K in bulk to 20 K in monolayer samples. From ab initio calculations, it was argued that magnetocrystalline anisotropy gives rise to an energy gap in the magnon dispersion, which suppresses low-frequency, long-wavelength spin waves (magnon excitations), and protects the magnetic order below a fnite Tc. A very interesting fnding of this study is the elevation of Tc much beyond the bulk value by using an ionic gate. In an ionic feld-effect transistor setup, LiClO4 dissolved in polyethylene oxide matrix covers both the Fe3GeTe3fake and the side gate. A positive gate voltage, Vg, intercalates lithium ions into the Fe3GeTe3 fake. Large variations in coercivity were observed by modifying the gate voltage. Additionally, the ferromagnetic Tc was raised to ~ 300 K as a function of doping. It was shown that the gate-induced electrons sequentially fll the sub-bands originating from the Fe d3z2, dxz, and dyz orbitals, resulting in sharp peaks in the DOS that

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FIGURE 6.4 Transport properties of 8 nm Fe3GeTe2 thin flm. (a) R-T curve, showing a metallic characteristic. The bottom inset is a ft to the variable range hopping model, ln(Rxx) ~ T-1/3. The scale bar of the top inset Hall-bar structure is 1 mm. (b) Angle-dependent anomalous Hall data. The measurement geometry is displayed in the inset. With the magnetic feld changing from perpendicular (θ  =  0°) to parallel (θ  = 90°) to the sample surface, the coercive feld increases largely, from which the easy axis can be determined to be out-ofplane. (c) Anomalous Hall data at different temperatures with the offset of 50 Ω. With rising temperature, the coercive feld decreases successively. Source: Reprinted with permission from Ref.42

correspond to the Fermi level passing through the fat band edges of the sub-bands, leading to sharp increase in Tc upon doping.

6.2.5 TRANSITION METAL PHOSPHOROUS TRICHALCOGENIDES (TMPCS) With a general formula (MPX3 ; M = Mn, Fe, Ni, Co; X = S) is a family of quasi-2D antiferromagnetic materials that has drawn immense attention in recent years.45–50 All these compounds are isostructural (i.e., monoclinic with C 2/m space group symmetry). In this family, transition metal ions are arranged in a honeycomb lattice structure in ab plane of individual layers, which are weakly held by van der Waals forces. The spins in each layer are aligned in the zigzag direction along

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the a axis,45,46 while the adjacent spin chains are anti-aligned (Figure 6.5 (a)), forming a zigzag antiferromagnetic order. Amongst them, MnPS3 is described by an isotropic Heisenberg Hamiltonian,51,52 FePS3 constitutes an Ising system, 51,53 NiPS3 by the anisotropic Heisenberg Hamiltonian,51 whereas CoPS3 appears to have an XY-like anisotropy.54 Thus, this family of 2D vdW materials form an excellent model system for studying 2D magnetism with spin S = 5/2, 2, 3/2, and 1 for MnPS3, FePS3, CoPS3, and NiPS3, respectively. Below Neel temperature (T N ~155 K), Ni ions in bulk NiPS3 are arranged in a honeycomb lattice structure in ab plane of individual layers, which are weakly held by van der Waals forces. The two-dimensional magnetic structure of NiPS3 comprises of ferromagnetic chains that are antiferromagnetically coupled in a zigzag manner. The moments in NiPS3 lie in the ab planes with spins in each layer aligned in ferromagnetic chains in a zigzag manner along the a axis,45,46 while the adjacent spin chains are anti-aligned (Figure 6.5), forming a zigzag antiferromagnetic order. Further inelastic neutron scattering studies and ftting of the data with linear spin wave theory using Heisenberg Hamiltonian with single ion anisotropy demonstrates the presence of small easy axis anisotropy and a magnon excitation gap of ~ 7meV.46 To study the thickness dependence of anti-ferromagnetic ordering, a later study investigated the temperature dependence of phonon spectra using Raman spectroscopy and quasi-electron scattering (QES) as a function of thickness down to the monolayer limit.48 It was observed that antiferromagnetic ordering persists down to two layers but is drastically suppressed in the monolayer. The Néel temperature, however, shows weak dependence on number of layers as long as it is greater than or equal to two. In comparison, in Ising-type antiferromagnet FePS3, antiferromagnetic ordering persists down to the monolayer limit.53 It is also in contrast to the case of ferromagnetic CrI3 or Cr2Ge2Te6, where the Curie temperature decreases as the thickness is decreased but remains fnite in the monolayer limit.2,3 These investigations indicate that the intra-layer exchange interactions are much stronger than the interlayer ones. In the monolayer samples, the monotonic increase in intensity of QES signal with decreasing temperature, even down to zero kelvin, suggests that spin fuctuations keep growing, destroying the antiferromagnetic ordering in accordance with Kosterlitz-Thouless (KT) transition. Recent studies have dedicated efforts in understanding the electronic and optical properties of this compound.49,50 From frst principles calculations, the zigzag AFM order was determined to be the lowest energy state for all values of the thickness in NiPS3 thin flms, whereas for single layer, the zigzag and Néel orders were found to be virtually degenerate (δE ≈ 0.2 meV/Ni).49 This facilitates large spin fuctuations and suppression of magnetic order in single layer. Additionally, it was shown that (Figure 6.5 (b)49) application of electric feld E ~ 0.7 eV/Å along z-axis induces a metal-insulator transition, with simultaneous vanishing of magnetic moments and electronic bandgap. A further photoluminescence (PL) study50 demonstrated the presence of highly anisotropic excitons (Wannier like) with optical anisotropic axis controlled by zigzag antiferromagnetic order. The strong exciton-magnetic coupling was revealed from the fact that the exciton emission spectra diminished and vanished near the Néel temperature (T N~155 K). The excitons also exhibit a strong dependence on thickness of the samples as the PL intensity fell off and

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FIGURE 6.5 (a) The magnetic structure of NiPS3 with the crystallographic unit cell and the unit cell used in the calculation of the magnetic dynamic structure factor. The insert shows the exchange interactions between the frst, second, and third nearest intraplanar neighbors. (b) Evolution of the electronic bandgap (crimson line) and the nickel magnetic moment (blue line) for monolayer NiPS3 with external applied electric feld. E is taken to be positive along the z direction and is shown schematically with the crystal structure of monolayer NiPS3. Source: Reprinted with permission from Refs46,49

peak blue shifts with decreasing layer number. Besides these, the observation of strong exciton-phonon coupling from linear dichroism (LD) studies pave the path for future investigations into exciton-magnon coupling in 2D materials.

6.2.6 TOPOLOGICAL INSULATORS Topological insulators (TIs) form a special class of materials that are insulating in the bulk but conducting at the surface. The surface states are bound by time-reversal symmetry (TRS), and the carriers are spin-momentum locked, which gives rise to intrinsic spin-polarized currents. TIs in two dimensions (2D) are described as a quantum spin Hall (QSH) phase, in which the spin-orbit interaction takes the role of an effective magnetic feld that acts in opposite directions for opposite spins, and TRS is preserved. QSH was frst investigated in graphene55 following the measurement of QH effect.56 However, it has since eluded detection experimentally due to its weak spin-orbit coupling and small bandgap (~ 10−3 eV).57 For stable 2D TI phases, strong spin-orbit coupling and a reasonably large bandgap are necessary. Some well-known topological insulators are Bi2Se3, Bi2Te3, Sb2Te3 and their solid solutions. Magnetic topological insulators (TIs) provide an important platform to explore emergent phenomena such as quantum anomalous Hall (QAH) effect,59,60 Majorana modes,61 magnetoelectric effect,58 etc. Magnetism arises in TIs by doping. For example, in Cr-doped (Bi, Sb)2Te3 thin flm, QAH was demonstrated experimentally.59 Linear magnetoresistance (MR), persisting up to RT, induced by 2D magneto-transport was reported in Bi2Se3 nanoribbons.58 For a magnetic feld parallel to the surface of a nanoribbon (a-b plane), the observed MR effect is negligible compared to the MR in perpendicular felds (Figure 6.6). The MR showed an |cos(θ)| angular dependence

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Source: Reprinted with permission from Ref.58

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FIGURE 6.6 2D linear magneto-resistance persisting to room temperature. (a) Resistance vs. magnetic feld of sample #2 from T = 2 to 300 K. The data in the perpendicular feld are shown as dots, while the data in the parallel feld are shown as solid lines for T = 2, 150, and 300 K. (b) R vs. rotation angle at T = 2, 150, and 300 K. The function |cos(θ)| is shown as solid red lines. (c) Conductance of Bi2Se3 nanoribbon sample #2 vs. rotation angle at T = 2, 150, and 300 K. Red solid lines represent the function |cos(θ)|, which is seen to describe the conductance data well for all temperatures.

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at arbitrary angle, illustrating the 2D origin of linear MR. Likewise, Mn doping of Bi2Se3, Bi2Te3 and Sb2Te3 thin flms grown by MBE gives rise to a large magnetic gap of ~90 meV at 1 K for Mn-doped Bi2Te3, which displays ferromagnetic hysteresis and out-of-plane spin texture.62 This is due to enhanced wavefunction overlap in a self-organized heterostructure of alternate Mn-Bi2Te4 and Bi2Te3 multilayers. On the other hand, Bi2Se3 with 6% Mn doping reveal only a temperature-independent nonmagnetic gap due to relatively weaker spin-orbit interaction in-plane magnetization and a TC of 6 K. Likewise, Cr doping (~ up to 12%) in epitaxial thin flms of Bi2Se3 grown by molecular beam epitaxy (MBE) show ferromagnetic hysteresis loops at 2 K.63 SQUID magnetometry and polarized neutron refectometry (PNR) measurements reveal a uniform magnetic moment ~ 2µB per Cr ion across the sample, with no enhanced magnetism near the surface or interface and a ferromagnetic TC ~ 8.5 K

6.3

SYNTHESIS AND CHARACTERIZATION OF 2D MATERIALS

Although magnetism can be induced in 2D materials by doping, defect engineering, interfacial effects, or other modifcations, it may not be stable for a long time and susceptible to thermal fuctuations in the absence of strong exchange interactions. For better control and stability, it is therefore necessary that 2D magnetic crystals with magnetic ions, which possess intrinsic magnetism, are grown with precise synthesis routes. Secondly, several theoretically predicted 2D magnetic systems have to be realized experimentally, especially those having TC around room temperature. Several well-established techniques such as chemical vapor deposition (CVD), chemical vapor transport (CVD), molecular beam epitaxy (MBE), pulsed laser deposition (PLD), wet chemical methods, exfoliation, etc. are used. Keeping in mind specifc characteristics desired and intended application, generally two approaches are employed to obtain 2D magnetic materials: top-down and bottom-up approach.

6.3.1

TOP-DOWN APPROACH

In 2D materials, different atomic layers are bound together by weak van der Waals (vdW) forces. Consequently, sheets with monolayer thickness can be easily separated from the bulk material by exfoliation either mechanically or by liquid phase exfoliation (LPE) without damaging the in-plane bonds of the layer. This forms the basis of top-down approach. Mechanical exfoliation uses mechanical forces to separate atomic layers/sheet, which can be easily achieved by using an adhesive substance such as a scotch tape. In fact, this was used to isolate graphene from graphite in the pathbreaking work of Novoselov and Geim.4 The bulk material like graphite, which is in the form of hexagonal sheets, is attached to the adhesive on the scotch tape and then peeled into a thin fake by using another adhesive surface or substrate. This process is repeated several times to get the desired thickness and further transfer the obtained sheets onto an appropriate substrate. Strong vdW interactions between Au and the top-most layer of bulk crystal/flm are often exploited for mechanical exfoliation of large area monolayers of TMDs.64 Several other 2D layered materials, such as h-BN,65 CrI3,2 Cr2Ge2Te6,40 Sb2Te3,66 NiPS3,67 etc. are also obtained through this method. Despite its nondestructive nature, absence of any chemical reaction, ability

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to obtain high quality crystals, there are some disadvantages, such as low production yield, labor intensive, low scalability, little control over microstructure, etc., which limits its use in various practical applications. To overcome these disadvantages, chemical exfoliation or liquid phase exfoliation (LPE) is used. In this technique, solvents are responsible for exfoliating the layered materials by intercalation, ion exchange, or shear force assisted exfoliation.68 The common intercalants utilized are acids/bases,69,70 oxidizing/reducing agents,71,72 etching agents,73,74 and some functional molecules.75–77 For effective exfoliation, the choice of solvent used depends on the strength of vdW forces between layers, surface energy, surface tension of the solvents, etc. All classes of 2D materials, including graphene,78 MoS2,79 h-BN,80 Cr2Ge2Te6,81 Bi2Te3,82 Bi2Se3,83 NiPS3,84 etc. can be easily obtained with this technique. LPE is a low-cost process and suitable for providing high yield of nanosheets of 2D layered materials. However, the produced sheets are of relatively poor crystal quality due to the damage caused by solvents and the shear forces used in this process.

6.3.2

BOTTOM-UP APPROACH

6.3.2.1 Chemical Vapor Deposition (CVD) CVD or CVT is a vacuum deposition technique to synthesize uniform thin flms over large areas. It is the most often used technique in semiconductor industry, specially in the process of atomic layer deposition. A typical process consists of controlled deposition of volatile precursors of desired material, which react and decompose on to a substrate. It facilitates layer by layer deposition of material on to a substrate. The deposition rate and the quality of flms is dependent on temperature, gas fow rate, and the distance between the precursors and the substrates. CVD method is preferable for making 2D ultrathin flms with relatively high crystallinity, conformal coverage, and controlled microstructure, shape, size, and thickness. It has been found very useful in synthesis and fabrication of graphene.85 Several other 2D materials, like h-BN,86,87 TMDs,88,89 NiPS3,90 and topological insulators,91 etc. with scalable size, precise thickness, and excellent properties were synthesized by this technique. CVD is a low-cost technique for producing thin flms with high growth rates, good reproducibility, and varied morphology. However, it is susceptible to contamination due to low vacuum and lacking precise control over thickness of flms. To overcome these limitations, molecular beam epitaxy (MBE) can be used. 6.3.2.2 Molecular Beam Epitaxy (MBE) It is a technique for epitaxial growth of thin flms and heterostructures, where one or more molecular or atomic beams of desired materials (e.g., gallium, arsenic) are generated by placing the source materials in heated evaporation cells. The substrate is heated to the necessary temperature and rotated to promote 2D and 3D growth mechanisms. The deposition is carried at ultrahigh vacuum (UHV) (10 −8−10 −12 Torr) and very small deposition rates (~ML per second) to ensure homogeneity and epitaxial growth. To monitor the deposition rate and thickness of the flms, refection high-energy electron diffraction (RHEED) is often used. Apart from growing single

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crystals, it has been used to grow 2D layers of high purity such as graphene on alumina and h-BN,92,93 MoSe2 on GaAs,94 Bi2Te3, Bi2Se362,63 as well. High cost, requirements of UHV, and very slow deposition rates are drawbacks from the point of view of high throughput and scalability compared to CVD. 6.3.2.3 Wet Chemical Methods In these methods, the precursors react with each other in a solution or liquid phase under controlled temperature, pressure, concentration, etc. For instance, hydrothermal/solvothermal synthesis is a common technique used for preparing thin flms and nanoparticles at typically lower temperature (100ºC to 250ºC). The precursors are dissolved in a solution, sealed off, and heated above boiling point in Tefon coated vessel. The high temperature and pressure stimulate chemical reaction and synthesis of high quality 2D materials.95–98 Likewise, 2D materials have been synthesized with template-assisted synthesis,99–101 microwave-assisted synthesis,102–104 etc. Since device applications need high level uniformity and crystallinity, to achieve better crystallinity and uniformity, post-deposition heat treatment is often required to obtain high-quality crystals. It is diffcult to monitor the growth of materials and control the dimensions of the end products limiting their use in 2D materials synthesis where high degree of control over the crystallinity and thickness is desirable.

6.3.3

CHARACTERIZATION TECHNIQUES

In 2D materials, due to quantum confnement, the magnetism may be destroyed or modulated by small electric or magnetic stimulus or simply due to thermal fuctuations. Thus, the characterization tools have to be sensitive yet subtle so as to not transform the virgin state of the material. Some of prevalent characterization techniques being utilized for probing the magnetic properties of 2D materials are vibrating sample magnetometry (VSM), superconducting quantum interference device (SQUID), magnetic force microscopy (MFM), magneto optic Kerr-effect (MOKE), magneto-circular dichroism (MCD), Raman spectroscopy, angle-resolved photoemission spectroscopy (ARPES), electron spin resonance (ESR), etc. Here we discuss working principles of a few of them. 6.3.3.1 Vibrating-Sample Magnetometer (VSM) VSM is often used to investigate magnetism in 2D vdW magnets.105–108 It is based on Faraday’s law of electromagnetic induction, where an electromagnetic force (emf) is generated in a pickup coil when there is a change in magnetic fux due to the vibration (up and down movement) of a magnetized sample near it. The induced emf gives rise to an electric current whose magnitude is proportional to the magnetization of the sample. For measurement in VSM, a sample is vibrated between a pair of pickup coils and a dc magnetic feld is applied to the sample. A lock-in amplifer can be used to detect the signal from the coils. The lock-in amplifer also can be used to give a dc voltage output, which is proportional to the magnetization of the material. The magnetization of the sample depends upon the strength of the magnetic feld and its magnetic susceptibility.

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6.3.3.2

Superconducting Quantum Interference Device Magnetometer (SQUID) It is a versatile tool for detecting and measuring extremely small magnetic felds (10 −15 T) and currents (10 −12 A) often associated with 2D vdW magnets.16,22,63,109 Its working principle is based on the tunneling of charge carriers (Cooper pairs) through a Josephson junction and quantization of fux in a superconductor. In the most widely used dc SQUID, two Josephson junctions are connected to each other in a dc biased superconducting loop. The dc current fows equally in both the arms. When an external magnetic feld is applied to the superconducting loop, a screening current opposing the change in magnetic fux circulates in the loop. The screening current is in-phase and out-of-phase in the two arms where the current is limited by the Josephson junction. When the current in any arm exceeds a critical value (i.e., Ic), a voltage develops across the junction. This voltage is read out as function of applied magnetic feld or magnetic fux Φa. Since the magnetic fux in a superconducting loop is quantized (i.e., φ  = h/2e), therefore, the critical current in the loop is modulated by the fux with a period equivalent to the fux quantum. Hence, the SQUID has a sensitivity of one fux quantum. In addition to the magnetic fux, it can measure electrical current (~ 10 −12 A) and electrical resistance (~10 −12 Ω). 6.3.3.3 Magneto-Optic Kerr Effect (MOKE) Magneto-optic Kerr effect is the phenomenon of rotation of the plane of polarization of a light beam on refection from the surface of a magnetized sample. MOKE can readily imagine domains in magnetic flms and 2D vdW sheets/layers at the relatively low resolution of ∼1 μm.2,3,41,109 The basic principle of this phenomenon is the circular birefringence due to differential refection (phase difference or rotation) of the incident radiation. For magnetization along the vertical z-axis (polar MOKE), an additional phase difference will arise between left and right circularly polarized photons traveling along the z-axis upon refection from the surface.110 For linearly polarized light, polarization direction rotates upon refection by an angle θK, known as the Kerr rotation angle. For most materials, the amount of rotation is small (~0.10 degrees) and depends on both the direction and the magnitude of the magnetization of the sample.111 6.3.3.4 Magnetic Force Microscopy Magnetic force microscopy (MFM) is a scanning-probe-based technique, in which a sharp metallic tip covered with a magnetic layer, such as Co-Cr, is utilized to probe local magnetic forces with high spatial resolution required in the case of 2D materials.18,112,113 It does not require any special sample preparation and can be done at ambient or temperature/environment-controlled conditions. This is a two-pass technique wherein the surface topography is measured in tapping mode during the frst pass, and magnetic forces are mapped on the second pass with respect to the topography data. The magnetic interactions between the super paramagnetic particles of the magnetic tip (Co-Cr) and the stray feld, a feld originating due to the ferromagnetic materials on the surface, will lead to defection of the tip as it experiences magnetic forces on a sample surface while scanning. The defection of the tip depends upon

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the strength of the local magnetostatic interaction between the tip and the magnetic features in the sample. The mapping of magnetic forces to topography AFM scans produces a 2D image with lateral resolution ~ 50 nm and can be used to characterize the domain structure of magnetic materials.

6.4

APPLICATIONS OF 2D MAGNETIC MATERIALS

To meet the ever-growing demands of high sensitivity, high packing density, low power consumption along with compatibility to existing CMOS architecture, 2D materials are considered as ideal candidates due to their unique physical properties. 2D vdW magnetic materials can be used in fabrication of spin valves, spin feldeffect transistors, magnetic tunnel junctions, Hall sensors, etc., to name a few.

6.4.1

SPIN VALVE

A spin valve is the fundamental building block of spintronic devices. It consists of two or more layers of conducting magnetic materials, whose electrical resistance can change depending on the relative alignment of the magnetization in the layers. A logical “off,” or the high-resistance state, is represented by the antiparallel confguration of two ferromagnetic layers, while logical “on,” or the low-resistance state, arises due to parallel alignment of ferromagnetic layers. Spin transport in graphene11 has been successfully demonstrated at room temperature and signifcant improvement in spin injection effciency in graphene through monolayer h-BN has been achieved in a later study.114 Atomically thin MoS2 based spin-valves (i.e., NiFe/ MoS2 /NiFe) show moderate MR of ∼0.4% at 10 K, which dramatically improved to 0.73% when the NiFe layer was capped with a thin gold layer.24 Further efforts are required to improve the spin-injection effciency, increasing the spin diffusion length and understanding spin fltering effects at the interface to improve the MR ratio.

6.4.2

HALL EFFECT SENSORS

These devices detect the presence and magnitude of small magnetic felds using the Hall effect. Graphene-based Hall sensors have been extensively investigated and performance evaluated due to its low carrier densities and high mobility, which promises extremely high magnetic sensitivity and low electronic noise, respectively. They show ultrahigh sensitivity of 2093V/AT with resolution of 1mG/Hz0.5 @ 3kHz115 and low noise (i.e., Hooge’s parameter αH < 2 × 10 −3) with sensitivity of 87 V/AT.116 Likewise, Hall sensors based on h-BN/graphene/h-BN van der Waals heterostructures show sensitivities up to 97 V/AT room temperature.117 Further, Hall sensor based on monolayer MoS2 on a hexagonal boron nitride (h-BN) thin flm under optimum bias conditions revealed a maximum sensitivity ~3000 V/AT.118

6.4.3

SPIN FIELD-EFFECT TRANSISTORS

A spin feld-effect transistor exploits the electrical control of spin current through the Rashba spin-orbit interaction, i.e., generation of an effective magnetic feld

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by an electric feld applied to the gate terminal to control the spin transport (via spin precession) from the source FM electrode to the drain FM electrode. At zero gate voltage, the transistor is in ON state since no spin precession results in spin direction parallel to the drain FM. Varying the gate voltage can alter the spin precession angle and result in a modulation of current across the spin transport channel. In spin-FET heterostructure of graphene and MoS2 , spin transport in the graphene channel could be switched ON and OFF by tuning the spin absorption into the MoS2 layer, by virtue of strong spin-orbit coupling of MoS2 , with a gate electrode.119 The spin transport can also be modulated by using spin lifetime anisotropy effect (SLA), as demonstrated in MoSe2/graphene120 and WS2/ graphene heterostructures.121

6.4.4

MAGNETIC TUNNEL JUNCTION (MTJ)

An MTJ consists of two FM electrodes with a thin insulating (dielectric) barrier sandwiched between them, where the electrons can tunnel through insulating barrier and cause charge fow. However, issues related to controlling the thickness of insulating barrier, its interfacial roughness, thermal and mechanical stability have limited the efforts for scaling up. In this context, 2D materials as tunnel barrier have huge potential due to their unique properties. Several 2D materials such as graphene,123,124 h-BN,125 MoS2,126,127 WS2128 have been investigated as tunnel barrier for MTJ devices (Figure 6.7). However, the magnetoresistance ratio (MR%) obtained from these have been relatively low ≤10% due to issues such as contamination during exfoliation, limited understanding of substate interactions, or due to reduced spin polarization in the buffer layer.

6.5 CONCLUSIONS AND OUTLOOK Magnetism in the 2D vdW materials is a fascinating subject where the role of spin-orbit coupling resulting in magnetic anisotropy is the key for long-range magnetic order. The weak interlayer coupling and fexible lateral confgurations of vdW materials, along with the tunability of magnetic properties such as magnetic anisotropy, transition temperature (TC), relative orientation of magnetic easy-axis through strain, electric feld, magnetic feld, doping, etc. is of intense scientifc appeal. Due to these unique physical properties and potential applications, 2D magnetic materials have received enormous attention in recent years. Exploring intrinsic magnetism in such materials is crucial not only from the point of view of understanding intricate low-dimensional magnetic interactions but also in the context of design and development of futuristic devices exploiting spin and charge degrees of freedom. Notwithstanding the current diffculties in experimental synthesis and characterization of 2D magnetic materials, signifcant research activity, emergence of novel materials heterostructures, and intriguing physical phenomenon await discovery in this nascent yet very stimulating feld in the foreseeable future.

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FIGURE 6.7 (a) The graphene-inserted magnetic tunnel junction (MTJ) device (top) and measured TMR characteristics (bottom) at temperature of 4 K and with low bias voltages (125 and 250 µV) applied. The CVD-grown graphene is transferred onto the 20 nm NiFe FM electrode. The arrows in the TMR graph indicate the relative orientation of the NiFe (lower) and Co (higher) FM contacts. (b) The Co/h-BN/Fe MTJ device structure is fabricated by annealing the Fe substrate, then directly growing h-BN on the Fe catalytic substrate via the low-pressure CVD technique (top), and fnally depositing the 15 nm Co top electrode by evaporation. The conductive AFM (atomic force microscope) measurement yields the resistance values of 105 Ω for the Fe substrate and 107 Ω for the monolayer h-BN/Fe stack. The relatively high TMR ratio of about 6% is reported in this work (bottom), arising from suppression of the FM electrode oxidation due to direct CVD growth of h-BN on top of the FM electrode. (c) Monolayer TMD is used as the spacer layer in the ferromagnetic sandwich structure to result in the spin valve effect. The NiFe/MoS2/NiFe device with monolayer MoS2 grown by CVD and transferred by the wet process (top) and the Co/WS2/NiFe device with the exfoliated single-layer WS2 (bottom) exhibit the magnetoresistance ratios of up to 0.73% and 0.47% at low temperatures, respectively. Source: Reprinted with permission from Ref.122

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7 Mechanical Properties 2D Materials

and Applications Deeksha Nagpal, Astakala Anil Kumar, Jashandeep Singh, Ajay Vasishth, Shashank Priya, Ashok Kumar, and Shyam Sundar Pattnaik CONTENTS 7.1 7.2

Introduction .................................................................................................. 124 Graphene as a 2D Material ........................................................................... 124 7.2.1 Mechanical Properties of Graphene ................................................. 124 7.2.2 Defects and Their Infuence on Mechanical Characteristics of Graphene............................................................................................126 7.2.3 Applications of Graphene ..................................................................127 7.3 Transition Metal Dichalcogenides (TMDs)...................................................127 7.3.1 Mechanical Properties of TMDs .......................................................127 7.3.2 Defects and Their Infuence on Mechanical Characteristics of TMDs.................................................................................................129 7.3.3 TMD Applications.............................................................................129 7.4 h-BN (Hexagonal Boron Nitride) as a 2D Material.......................................130 7.4.1 Mechanical Properties of h-BN .........................................................130 7.4.2 Defect and Their Infuence on Mechanical Characteristics of h-BN .....................................................................131 7.4.3 Applications of h-BN .........................................................................132 7.5 Black Phosphorus as 2D Material .................................................................132 7.5.1 Mechanical Properties of Black Phosphorus.....................................132 7.5.2 Defect and Their Infuence on Mechanical Characteristics of Black Phosphorus ..............................................................................132 7. 5.3 Applications of Black Phosphorus.....................................................133 7.6 MXene as a 2D Material................................................................................133 7.6.1 Mechanical Properties of MXene......................................................133 7.6.2 Defect and Their Infuence on Mechanical Characteristics of MXene ...............................................................................................134 7.6.3 Applications of MXene......................................................................136 7.7 Other Materials..............................................................................................136 7.8 Conclusions and Future Prospectus...............................................................137 References...............................................................................................................137

DOI: 10.1201/9781003247890-7

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124

7.1

Emerging 2D Materials and Applications

INTRODUCTION

In recent years, 2D materials attracted researchers’ attention due to interesting mechanical properties such as fexibility, mechanical strength, elastic modulus, and the friction transfer between the different layers. The 2D materials can be analyzed for their myriad faws and functionalization in numerous ways using the methodologies of synthesis, isolation, and propagation. Exfoliation and chemical vapor deposition are two common techniques used in the synthesis of layered 2D materials. The mechanical characteristics such as brittleness and fexibility of 2D materials are responsible for the distortion out of the plane in terms of angstroms. However, they have a big impact on the effective characteristics. Ripples or corrugations are the terms used to describe these little out-of-plane deformations. These are distinguishable from wider buckling deformations, such as wrinkles or crumples, that can occur from 100 nm to several microns as a result of contact with another substrate [1–2]. Addressing the functionality of 2D materials in actual devices, their durability during ambience and varying environments, is required for practical application. The investigation on the dependability of 2D materials based systems on electrical or thermal stress assessment methods along with device stability requires attention of the researchers. The device safety, performance and the impact of the device is an important aspect of fabricating a device. The characteristic material used in the device fabrication plays a key role in device performance. Currently, the 2D materials that are used in several mechanical devices will have no adverse impact on the environment and are expected to enhance the device performance. In the mechanical viewpoint, 2D material structure attributes to its unique layered structure with good fexibility and high mechanical strength, making it an attractive active material in the fabrication of several mechanical devices such as sensors, piezoelectric devices, etc. It is possible to alter the mechanical behavior of the material with the incorporation of defects/functional groups [3]. According to Hooke’s law, the strain developed under the application of stress will have the linear relationship, which may be regarded as a mechanical comparable to Ohm’s law and is among the most underlying mechanical characteristics of mechanical components. If we introduce a uniaxial tensile or compressive stress to the substance, then the equation is followed as σ = E ε, where σ, E, ε represents stress, modulus of elasticity, and strain, respectively. Hooke’s law may be used to describe both bulk quantities and 2D nanosheets. To analyze the in-plane characteristic of the elastic constants of a monolayer, Hooke’s law is extensively applied. The ratio of applied stress in two different orientations is termed as the Poisson’s ratio “m” [4, 5]. Surface adhesion is a fascinating feld of research and an important mechanical characteristic of 2D materials. Layers with an atomic thickness and unequalled fexibility can adapt to contacting surfaces far better in comparison to bulk materials, which leads to signifcantly improved contact area and stickiness [6].

7.2 7.2.1

GRAPHENE AS A 2D MATERIAL MECHANICAL PROPERTIES OF GRAPHENE

The interest in nanoscience has grown among academia and industry since the discovery of monolayer graphene. Graphene possesses remarkable mechanical and functional properties and has signifcant potential to unveil new devices in the market.

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The development of improved devices is becoming a priority among researchers all around the world. However, creating graphene-based products, particularly highquality and well-structured graphene in large quantities, becomes a signifcant diffculty. As a result, researchers continue to be concerned about manufacturing high-quality graphene, and many production methods have been used to manufacture high-quality graphene. Graphene is the origin of all graphitic forms. As a result, all carbon compounds and graphene-related materials have the same fundamental chemical structure. The sp2 hybrid bonds cluster the carbon atoms symmetrically in a hexagonal lattice to form a thick carbon layer. Sensors, actuators, electronics, and biomedical devices are just a few of the sectors where graphene-based materials and composites provide incentives. In the natural world, the thinnest feasible 2D membrane is monoatomic graphene. As a result of its great strength and ductility, it is ideal for a wide range of applications. Studies of graphene in various matrices have shown its high stiffness and strength under tension while also revealing the complete stress-strain response under compressive loads. Denser graphene has a nonuniform strain distribution in the outof-plane direction, which reduces its elastic properties. It’s worth to note that boundary dislocations operating on graphene in air cause wrinkling because the buckling instabilities threshold for such a thin plate is so low. Graphene and its related materials have outstanding mechanical properties, making them a suitable material for engineering components. Graphene’s thickness has no effect on its in-plane elastic modulus. As a result, single-layer graphene was predicted to have the same in-plane elastic modulus as bulk graphite. Signifcant differences owing to ripples and other variables, on the other hand, need more sophisticated theoretical models for improved consistency [7]. The hexagonal lattice is developed by their stable sp2 bonds, which restrict various in-plane deformations. In suspended monolayers of graphene, 1 nm ripples height with less than 25 nm periodicity have been observed as a result of temperature variations and elastic stresses [8]. The graphene has a negative thermal coeffcient with decreased in-plane stiffness due to its non-fat arrangement [9, 10]. The out-of-plane wrinkling in graphene may result in reduction of the elastic modulus of a single-layer graphene while increasing its toughness or chemical activity. The width of a monolayer of a 2D material is usually considered as the interlayer thickness. The single sheet of graphene is often regarded as 0.334 nanometers thick, while atomic force microscopy measurements have shown that the thickness varies between 0.4–1.7 nm. [11]. The 2D membranes are also evaluated for their mechanical behavior and linearity using bend stiffness, which is related to ripple production and electron scattering. Flexible and transparent electronics, as well as corrosionresistant coatings, might beneft from chemical vapor deposition (CVD) graphene [12–15]. When graphene purity and quality are not a major concern, exfoliation and reduction of GO seem to be the best methods for producing vast amounts of graphene at minimal cost. Exfoliated graphene might be utilized in conductive coatings, inks, energy storage, and lithium-ion batteries [16–18]. The heat generated in graphene-based systems raises the temperature, which has a negative impact on graphene’s mechanical characteristics. Figure 7.1 (a) explains that the increase in temperature results in the linear reduction in Young’s modulus [19]. The stress-strain relationship is linear at smaller strains, but then stress grows

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FIGURE 7.1 Mechanical characteristics of graphene. (a) Temperature-dependent change in Young’s modulus [19]. (b) For varied percentages of silicon doping, the stress-strain curve of silicon-doped graphene with zigzag loading [20]. (c) Data comparison of stress-strain curve [5, 21].

nonlinearly with strain until it reaches its peak, known as maximum tensile stress, at which time the material severely breaks and undergoes brittle breakdown. It is clear from Figure 7.1 (b) that the stress-strain curve exhibits catastrophic fracture even after doping graphene by silicon atoms [20]. In addition, Figure 7.1 (c) analyzes the failure stress associated with an expected failure stress of 42.4 Nm−1.

7.2.2

DEFECTS AND THEIR INFLUENCE ON MECHANICAL CHARACTERISTICS OF GRAPHENE

The density of the faws, as well as the layout of the faults, impact mechanical properties. In the ideal crystal structure, deformations and disclinations affect a crystal’s mechanical properties. Defects can impact the shear strength, fracture toughness, durability, and mechanical characteristics of crystals in general, and it’s important to fgure out how to recover, characterize, and utilize these impacts. Atomic force microscope (AFM) nanoindentation can be used to detect the effect of defects on mechanical properties [22]. It is found that carbon tends to establish an interlayer, connecting around faws, in irradiated multilayer graphene, which partially restores the damaged modulus. Multilayer graphene has a greater elasticity than monolayer graphene as an outcome of this interlayer bonding, which makes it more resistant to radiation. The CVD graphene is an excellent technique for investigating the impact of faws and surface defects. The Young’s modulus and compressive strength of CVD graphene are lower than those of softly exfoliated graphene [23–25]. Graphene produced through CVD process is prone to defects and grain boundaries. Point defects reduce graphene’s elastic fexibility and compressive strength, while grain boundaries defects can increase or decrease graphene’s strength according to their tilt

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angles [26–29]. The harmonic assumption was utilized, with coeffcients indicating force constants, to construct discrete deformations in graphene when generalized for eigen deformations [30]. An analytical system can be more easily addressed with the help of discrete Fourier transformation. Core values and structures were assessed in accordance with published data. The fracture toughness simply declined with the defect density, in contrast to the behavior of the modulus of elasticity, and this may be handled by typical fracture continuum models.

7.2.3 APPLICATIONS OF GRAPHENE In spite of possessing the dimension in atomic scale, the 2D materials show better mechanical strength and endurance, allowing them to be used in a variety of disciplines. Consequently, 2D materials are not only used as additives to strengthen them but also used in extremely sensitive resonators and versatile electronic devices. There are a few 2D materials that possess pressure-dependent functionalities, such as variations in electrical and chemical characteristics, that can be used to produce chemical monitors and piezoelectric sensors. Graphene resonators are the most extreme 2D nanoscale resonators since the thickness is of atomic order and the addition of few atoms increases the signifcant fraction of the volume of graphene [31]. The graphene exhibits a higher surface to volume ratio with higher value of Young’s modulus than existing microelectromechanical system (MEMS) resonators. As a result, graphene may be used to achieve high-frequency and high-Q resonators. The MEMS voltage-controlled oscillators as well as quantum information systems are two more uses of graphene nano-mechanical resonators [32–33]. Furthermore, graphene resonators show dynamic intermodal coupling, making them attractive in quantum information technology. Graphene has cleared the direction for future usage in engines, actuators, detectors, energy harvesters, and resonators, among other things. Graphene is chemically inert, has elevated thermal stability, and is resistant to gas diffusion. This makes graphene superior to any other material in protecting metals from surface oxidation. The copper (Cu) and copper/nickel (Cu/Ni) alloys can be protected from air oxidation by using techniques such as CVD-grown graphene sheets [34]. In addition to corrosion resistance, graphene might be used to create a variety of hybrid coatings for electronics, including self-cleaning or anti-fouling coatings and fre-resistant coatings. The 2D materials are suitable for adaptable electrical devices because of their ultrathinness, great elasticity, and robustness. There are no performance degradation issues with graphene feld-effect transistors on adaptable substrates at high loads of up to 5%. Graphene was chosen for medication delivery, gene therapy, genetic analysis, and tissue engineering because of its porous structure and stable chemical characteristics.

7.3 7.3.1

TRANSITION METAL DICHALCOGENIDES (TMDs) MECHANICAL PROPERTIES OF TMDS

The TMDs such as tungsten disulfde (WS2), molybdenum disulfde (MoS2), and tungsten di-selenide (WSe2) have been in attention due to their favorable energy gap and also

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due to their transition near the monolayer from indirect bandgap in bulk to direct bandgap [35]. The mechanical characteristics of TMD coatings are aimed toward less-defective, exfoliated MoS2 fakes. The MoS2 has an overall breaking strength of 15±3 N/m (23 GPa), which has been several times lower than that of monolayer of graphene but in contrast to steel has been higher. There seems to be no defectrelated Raman spikes in 2D TMDs [36]. Using frst-principles density functional computations, the mechanical characteristics of many additional TMDs were also studied. Monolayer of MoS2 has been associated with 9.61 eV bending modulus, whereas graphene has a bending modulus of 1.4 eV, which is very less in comparison to the monolayer of MoS2. This difference in the value of bending modulus has been due to the fact that monolayer of MoS2 has been associated with three atomic planes in comparison to the graphene [37]. The frst-principle technique was used to determine the bending rigidities of different TMDs that would not require empirical factors such as thickness [38]. The elastic mechanical characteristics of the MoS2 nanosheets were measured by applying a force in the middle of the suspended area with an atomic force microscope tip. When the tip and material come into contact, the deformation of the nanosheets, the defection of the atomic force microscope cantilever, as well as the movement of the atomic force microscope instrument’s monitoring piezo tube are all linked. When the tip and material come into contact, the deformation of the nanosheets, the defection of the atomic force microscope cantilever, as well as the movement of the atomic force microscope instrument’s monitoring piezo tube are all linked. Elastic deformations are calculated by the equation: δ = △Zpiezo- △Zc

(1)

Where △Zpiezo is the displacement of piezo-tube, and △Zc is the fuctuation of cantilever. MoS2 nanosheets have a very signifcant value of Young’s modulus, E = 0.33 ± 0.07 TPa [39]. Figure 7.2 depicts the elastic characteristics of two-dimensional monolayers and heterostructures [40].

FIGURE 7.2 Mechanical properties of TMDs: (a) Representation of the indentation analysis. (b) Illustration of an AFM topography of a monolayer of MoS2 by taking scale bar 500 nm. (c) Curves of force-displacement for distinct chemical vapor deposited monolayers and heterostructures [40].

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TMDs materials must be grown on a huge scale before they can be used in actual devices. The most effcient technique to accomplish massive growth is by chemical vapor deposition (CVD). The sulfurization of metallic thin flms and the vapor phase reaction of metal oxides containing chalcogen precursors are the two types of this technique.

7.3.2

DEFECTS AND THEIR INFLUENCE ON MECHANICAL CHARACTERISTICS OF TMDS

MoS2, one of the most prevalent 2D semiconductors, has a grain boundary arrangement that may be exploited to change its mechanical properties. According to DFT calculations, the defect production energy of grain boundaries in polycrystalline MoS2 is less than that of the grain core. This grain boundary (GB) model’s estimated formation energy is about 2.50 eV/unit, which is fairly comparable to that of its crystallographic counterpart (2.53 eV/unit), indicating that grain boundary creation is reasonably straightforward [41–42]. The TMDs are made by directly sulfurizing a metal oxide thin layer, but there are certain drawbacks, such as the complications in controlling the width of the predeposited metal oxide, which impacts wafer-scale homogeneity.

7.3.3

TMD APPLICATIONS

Before utilizing TMDs materials in real devices, they must be produced on a large scale. Chemical vapor deposition (CVD) is the most effective method for achieving enormous growth. The single-layer MoS2 can be used in a wide range of applications, including composite reinforcement and the manufacturing of fexible electronic equipment. Similar to graphene feld-effect transistors, devices constructed of TMDs on adaptable substrates demonstrate great stability and better performance. The development of feld-effect transistor with the MoS2 as an active material exhibited a better on/off ratio of 107 and a carrier mobility of 30 cm 2/ Vs at a moderate bending radius of 0.75 to 1 mm without signifcant degradation [43–44]. The MoS2 has a strain-dependent electronic structure that may be utilized to control excitons by localized strain. It has been demonstrated that, under localized strain, the restricted point of MoS2 may focus and gather excitons from adjacent MoS2, acting similarly to a funnel, which is utilized to enhance the effciency of solar power output and light emitters [45]. Aside from that, the MoS2 drum resonators had a resonance frequency of up to 60 MHz and a Q factor of approximately 3,000 [46]. The sensor based devices and nanogenerators have been developed by using the piezoelectricity of 2D materials. The MoS2 was used to fabricate strain/force monitors with gauge factors in excess of 1,000 [47]. An extremely high level gauge factor has been obtained on changing the Schottky barrier width with the help of piezoelectric polarization by doing the strain variation. The MoS2 piezoelectricity has been proposed as a method of enhancing the theoretical effciency limit of MoS2 solar panels [48]. The application of the piezoelectric photocatalyst such as zinc oxide, zinc sulfde, and molybdenum disulfde nanostructures in the

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degradation of methylene blue dye is successfully reported by Fu et al. [49]. It is observed that the degradation effciency is almost 2.5 times more as compared to the material combination without piezoelectricity. Interestingly, the effciency remains around 80% after the fve exhaustive cycles. Han et al. [50] fabricated the piezoelectric nanogenerator with the layered MoS2, which is passivated with the activated sulfur vacancies. The sulfur vacancies reduce the carrier concentration on the surface of the MOS2 resulting in the prevention of piezoelectric polarization by free carrier charges. The piezoelectric output voltage is found to increase signifcantly by a magnitude of 3−, 2− times for sulfur treated and nanosheet piezoelectric nanogenerators, respectively. Similarly, the MOS2 nanofakes were studied as an active material in piezoelectric nanogenerators by Wu et al. [51]. Interestingly, it is observed that the single monolayer of MoS2 strained by ~ 0.53% provides an output voltage of 15 mV and the current of 20 pA with effciency of 5.08%. Further research will have to be performed on the application of various other transition metal dichalcogenides as theoretical results predict better performance of these materials

7.4 7.4.1

H-BN (HEXAGONAL BORON NITRIDE) AS A 2D MATERIAL MECHANICAL PROPERTIES OF H-BN

The h-BN is a white powder that resembles the hexagonal confguration and has a layered structure coating that can be used to alleviate the in graphene problem by opening up the graphene bandgap and therefore increasing the switching functioning of graphene devices. Because graphene’s layered heterostructures are advantageous for fexible and transparent devices, its mechanical characteristics are signifcant. According to nano-indentation studies on h-BN over the micrometer scale implicated the mechanical behavior of h-BN as an equivalent to those of graphene [52]. According to atomic force microscopy bending observations and classical plate theory, the folding modulus of h-BN improves with lowering thickness, approaching the expected value of C33, which refects elasticity underneath a longitudinal load with transverse displacement [53]. The thickness dependency of the bending modulus is caused by the layer pattern of stacking faws. The mechanical properties of hexagonal boron nitride nanosheets (h-BNNS) persist reasonably constant and are related with the ratio of radius of circular-region and nano-indenter. The mechanical properties of (h-BNNS) progressively deteriorated with enhanced temperature. The loading force versus defection graphs in six and eight layers of h-BNNS reveal the phenomenon with sudden reductions, which might be related to the multilayered structure’s transitional deformation. Despite the fact that the higher load force increases because the count of layers increases, the enhanced gradient of higher load force decreases or remains generally steady. Young’s modulus for h-BNNS grows from single to two layers, then drops after three layers, and ultimately reaches about 0.7 TPa, which is less as compared to the single-layer h-BNNS Young’s modulus. Similarly, the shift in fracture toughness shows the similar pattern, with higher

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FIGURE 7.3 Effect on h-BNNS (a) loading-force vs. defection curve. (b) Maximum loading-force. (c) Young’s modulus fuctuates with successive layers. (d) The fracture strength varies under various layers [54].

and lower fracture strengths of 145 GPa and 115 GPa, respectively, as shown in Figure 7.3 [54]. The inherent hardness of h-BN sheet with grain boundaries on the surface displayed a clear linear relationship with the surface’s infection angles, indicating that the intrinsic strength dropped linearly as the infection angles rose.

7.4.2

DEFECT AND THEIR INFLUENCE ON MECHANICAL CHARACTERISTICS OF H-BN

The h-BN sheets shattered at or around the contact, suggesting that the interface was weaker for that material. Due to lattice distortion, faults are invariably formed at the interface during the epitaxial growth of h-BN heterojunctions using the epitaxial CVD technique [55–56]. Furthermore, faws may be utilized to carefully regulate the device’s electrical structure, which infuences device’s band structure, carrier density, and spin properties. The stress on a particle progressively increases during tensile crack growth, and the stress concentration is present at the graphene-h-BN interface. The frst bond deformation of the C-B bond at the surface occurs when the tensile distortion achieves the fracture strain. Afterward the bond fracture expands across the interface direction and induces breakage to the opposite side of the interface unless entire fracture occurs [57].

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Emerging 2D Materials and Applications

APPLICATIONS OF H-BN

Versatile “All-2D” devices fabricated by using only 2D materials such as h-BN, MoS2, and graphene have shown the high value of carrier mobility and on-off rate of 45 cm2/Vs 107, respectively, and opacity of 88% to 95% while tolerating 1.5% to 2% strain [58]. The h-BN nanosheets have already been extensively used as nanofllers in polymers to improve their ability. The thermal conductivity of polymethyl methacrylate (PMMA) lowers from 184 to 160 ppm0C−1 at 0.3 wt% h-BN nanosheet concentration, and the modulus of elasticity improves by 22%. Due to the nonaligned character of the nanocomposites, it has been found that the nanoparticle strength is susceptible to the dispersion of nanofllers and that a relatively modest amount of reinforcement is obtained. Other uses for 2D h-BN nanosheets include sensing devices for methane, ethanol, and NH3 sensors; separation of multiple impurities in oil, solvents, dyes, and trihalomethanes to clean and remove impurities from water; etc.

7.5 7.5.1

BLACK PHOSPHORUS AS 2D MATERIAL MECHANICAL PROPERTIES OF BLACK PHOSPHORUS

The growing interest in new developing single-element structured material is due to black phosphorus’s layer reliant 0.3–2.0 eV bandgap modulation region, which is similar to that of transition metal dichalcogenides and single- and few-layered graphene [59]. Because of its application in various sectors, ongoing improvements are being made to enhance the productivity of phosphorene thin flms. The phosphorene has unusual mechanical characteristics, making it a potential new material for nanomechanical devices and systems [60]. It can withstand tensile stresses of about 30%, which is greater than the values recorded for graphene or single-layer MoS2. Because of its distinctive puckered structure, phosphorus exhibits a negative Poisson’s ratio [61]. The Young’s modulus of this material is less than that of graphene as well as single-layer MoS2 [62]. Phosphorene has the capability to be an effective fller for reinforcing polymers and composites due to its exceptional mechanical characteristics. Polyvinylchloride following phosphorene reinforcement has a signifcant increase in all mechanical measures like Young modulus, composite stiffness, and tensile hardness, making it a strong contender for graphene reinforcement [63]. The black phosphorus experiences crystal structural changes under high pressures, frstly from an orthorhombic to a rhombohedral structure at approximately 5 GPa and subsequently to a basic cubic structure around 10 GPa [64]. An investigation of black phosphorene stress-strain correlations under uniaxial tension was conducted. Increasing the temperature of black phosphorene causes its Young’s modulus to decrease along with its fracture toughness and fracture strain [65–66].

7.5.2

DEFECT AND THEIR INFLUENCE ON MECHANICAL CHARACTERISTICS OF BLACK PHOSPHORUS

During the development of 2D phosphorus sheets, interstitial faults are common. Self-interstitial phosphorus has a substantially lesser formation energy than graphene. The main dislocation of phosphorene is 5/7 pairs with such a buckled confguration,

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identical to hexagonal 2D materials. The tilt angle controls the grain boundary (GB) energy in phosphorene, which is made up of an assembly of dislocations. Because the production energy of the deformations that make up GBs in phosphorene are vastly shorter than those in graphene, the GBs in phosphorene seem to be more reliable than that in graphene [67]. The vacancies in phosphorene did not remain static; they migrated and aggregated, producing line defects that were much more energetically advantageous. The mobility of faws was strongly infuenced by temperature, with single vacancies moving 16 orders quicker at ambient temperature than those at 70 K [68]. The Poisson’s ratio of black phosphorus is relatively low, which restricts its use in equipment due to poor fracture toughness. The fracture process is investigated using molecular dynamics, which leads to the conclusion that stretchable fracture developed across the X axis is produced by the breakdown of interfacial bonds across the Y axis [69].

7.5.3

APPLICATIONS OF BLACK PHOSPHORUS

This unusual mechanical anisotropy may be used to make nano-mechanical resonators, thermoelectric appliances, and motion detectors with adjustable functionalities that aren’t possible with isotropic materials. The 2D black phosphorus has been utilized in a nanoelectromechanical resonator, exhibiting sustained resonance vibrations at extremely high frequencies, whereas the fundamental beneft of mechanical anisotropy may be harnessed to enable novel phenomena that are not possible in isotropic 2D materials [70]. A potential use for two-dimensional black phosphorus resonators is mass monitoring or environmental condition assessment. As the environment deteriorates, structural and mechanical characteristics might be affected, resulting in a readily visible change in the resonant frequency of the unique modes.

7.6 7.6.1

MXene AS A 2D MATERIAL MECHANICAL PROPERTIES OF MXENE

Through selective peeling of the aluminium layer in Ti3AlC2 phase using hydrofuoric acid, a novel 2D transition metal carbide and nitride (MXene) was created, which sparked a lot of attention [71–74]. Exfoliation of the MAX phase (abbreviated later) that is frmly bound inside a 2D layer and kept together through weak van der Waals contact may be used to make MXenes. The MAX phase formula is Mn+1AXn (n = 1, 2, 3), where M is a transition metal, A is a group IIIA or IVA element, and X would either be a carbon or nitrogen atom [75–76]. With increase in the value of n, the production of MXene and MAX may be reduced. The Tin+1Nn has a greater production rate than Tin+1Cn, implying that nitrogen-containing MXene has a lower durability than carbon-containing MXene [77]. The MXene material possesses metal and ceramic properties, with great stiffness and high strength. Corrosion resistance, strong electrical conductivity, and good thermal conductivity are all features of this material. MXene also exhibits high hydrophilicity, adaptability, and plasticity and has increasingly become a 2D materials research focus. According to experimental data, the modulus of elasticity of monolayer Ti3C2 (OH)2 is around 300 GPa, which is

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FIGURE 7.4

MXene (Ti3C2Tx) Young’s modulus compared to other 2D materials [81–85].

lower than graphene, equivalent to several transition metal carbides, but larger than most oxides and multilayer clays [77]. The bonding here between M2X and the terminating group improves the mechanical characteristics. Single-layer M2X, in-plane elasticity is estimated to be between 42 and 199 N m−1, that is less stiff as compared to graphene (341 N m−1) and monolayer h-BN (276 N m−1) [78–80]. Young’s modulus of MXene (Ti3C2Tx) is observed as 0.33 ± 0.03 TPa [84], which is more than the phosphorus (0.166 TPa) [85], as demonstrated in Figure 7.4. However, h-BN has a higher value of Young’s modulus in the comparison of other 2D materials.

7.6.2

DEFECT AND THEIR INFLUENCE ON MECHANICAL CHARACTERISTICS OF MXENE

In contrast to the deformation process, in sensor application, primarily the resistance between the Ti3C2Tx lamellae changes due to hydrogel distortion caused by an external force, converting the mechanical signal to an electrical signal [86]. The Tx refers to the surface terminations produced during the etching process, such as hydroxyl (-OH), oxygen (-O), and fuoride (-F) [87]. Substantial fake size is preferred for mechanical reinforcing operations as a flm will have even less sheet-to-sheet contact defects, and the considerable in-plane stiffness of the nano-components will be fully used. The bending strength is a key parameter that describes the mechanical

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FIGURE 7.5 (a) Force-defection (F-δ) curves comparison for monolayer graphene and Ti3C2Tx substrates [81]. (b) The F-δ curves of a bilayer Ti3C2Tx fake at various loads. The AFM picture of the broken membrane is shown in the top corner [84]. (c) Inside the linear elastic regime, the loading and unloading curve [90].

behavior of 2D materials undergoing bending deformations [88]. Some factors, such as flm thickness and the functionalization component, have an impact on bending rigidity. Bending capability of Ti2C is lesser in comparison to MoS2 because of differing atomic confgurations and a higher thickness in MoS2 than Ti2C [89]. The comparison of force versus defection curves (F-δ curves) for suspended Ti3C2Tx with graphene monolayers is made as graphene is a superior 2D material (Figure 7.5 (a)). The fgure depicts the membrane’s behavior at the start of the indentation procedure. Before snapping, linear force versus defection dependency extends till it crosses the arc and assumes this location to be the origin, which would be required to achieve the right F-δ correlation. The trajectory of extension and retraction in each loading cycle, as well as the curves for load changes, are retraced. The MXene fakes have a great fexibility, and no fake breakage takes place during the assessments. This bilayer’s fracturing ruptured at a load force of around 200 nN and a defection of about 38 mm as shown in Figure 7.5 (b) [84]. The linear regimes are not fully elastic, shown by the stress-strain curves. The loading and unloading lines do not coincide when the Ti3C2Tx sheet is compressed within its linear elastic range and subsequently unloaded, suggesting that the linear distortion is viscoelastic instead of simply elastic, as shown in Figure 7.5 (c). Unless and until a flm is stretched beyond the linear elastic regime, material undergoes

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plastic deformation, under which the strain increases faster than the stress, implying that sliding between neighboring sub-layers is likely to play a dominating role in the irreversible process of deformation [90].

7.6.3

APPLICATIONS OF MXENE

These have been utilized in a variety of applications, including power storage, catalyst supports, methanol oxidation and hydrogen transformation, supercapacitors, and more [91–94]. Around 2013, MXene seems to have been a major research area in the battery sector. Due to its controlled production technique and remarkable characteristics, MXene has also shown potential in wearable sensing applications. The MXene-derived photodynamic and photothermal compounds have a high hydrophilic nature, which facilitates excellent dispersion and sustainability in physiological fuids. With the polar terminating groups, anticancer medicines may be readily grafted upon MXene surfaces. Optimizing the swelling ability of MXene hydrogels can greatly improve the quality of anticancer medicines with loading capabilities as greater as 84% and signifcant release percent [95]. Because pure MXenes, like all the other 2D materials, are not stretchy, introducing polymers to them can improve their mechanical robustness and sensing range [96]. The Ti3C2Tx nanosheets created by MXenes and poly(diallyldimethylammonium chloride) (PDAC) from the original Ti3C2Tx MAX phase are used in layer-by-layer assembly to make composites. The flm’s conductance can exceed up to 2,000 S/m. Composite membrane strain sensor on polydimethylsiloxane (PDMS) could bent up to 40%, whereas the bending sensor on polyethylene terephthalate (PET) can be stretched to 35% [97]. Furthermore, MXenes are immediately added to the produced elastic substrate, and the high conductivity of MXenes, as well as the superior mechanical qualities of the stretchy substrate, are exploited to meet the resistance impact and geometric features of the adjustable piezoresistive sensor. The MXenes can be useful in detecting the strain with the help of crack mechanism. When the MXenes-based monitor is fractured by strain, the characteristics of the monitor changes as the crack size rises.

7.7

OTHER MATERIALS

Silicene, like graphene, is a hexagonal 2D material with composition of silicon [98]. Despite the better characteristics of silicene and the need for high similarity with the silicon-based industries, silicene’s high reactivity quickly promotes the spontaneous dissociation of oxygen on the interface and the creation of the Si-O bonding. Because silicene is so unstable in its natural environment, determining its mechanical characteristics empirically is exceedingly challenging [99–100]. The Poisson ratio and in-plane stiffness of silicene are estimated to be 0.3 and C = 62 J/m 2, respectively, by applying the calculated changes in elastic energy to a quadratic deformation model [101]. The mica is a silica-based material that has a complex composition and is a two-dimensional substance. Multilayered mica has a 190 GPa elastic modulus as determined by AFM [102]. Mica also has a minimal pre-strain of 0.25 N/m and excessive breaking force of 4 to 9 GPa. Mica’s mechanical characteristics suggest

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it might be used as an adjustable extremely thin insulating/dielectric substrate. In recent years, research has focused on the formation of moiré patterns on the various 2D materials and their effect on the chemical/ physical properties. The van der Waals defects, or interlayer deformations, can be used to quantify Moiré patterns in actual materials systems [103]. Throughout the lattice stackings, these patterns provide areas of commensurability and incommensurability. Moiré textures on h-BN, SiC, and faces of face centered cubic (FCC) metals have been seen experimentally.

7.8

CONCLUSIONS AND FUTURE PROSPECTUS

Many alternatives are emerging at the convergence of 2D materials mechanics. Recent advances in the study of mechanical characteristics of 2D materials, as well as their applicability as a specialized type of mechanical system, have been summarized. Nature’s strongest 2D substance has been confrmed to be graphene. Other 2D materials, like MoS2, offer comparable mechanical properties to traditional materials. The anisotropic mechanical properties of 2D materials with asymmetric in-plane lattices, such as black phosphorus, would be considerably more diffcult to study but will be highly intriguing. The mechanical characteristics of 2D materials have been observed to decline as the number of layers increases, and defects can exacerbate this decline. When it comes to the impact of faws, the effect of grains and their related properties on the strength/durability of 2D materials is currently being debated. It is necessary to develop synthesis/deposition methodologies for 2D materials with well-controlled grain sizes and orientations in order to overcome these challenges. Defects can strengthen or diminish interlayer interactions, affecting the mechanical characteristics of the system. Both in terms of measurements and usage, there have been signifcant advancements. Nevertheless, there are still unanswered issues and problems in this area of study. Except for graphene, research corresponding to the mechanical behavior of the other 2D materials is to be investigated.

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96. Zheng Ling, Chang E. Ren, Meng-Qiang Zhao, Jian Yang, James M. Giammarco, Jieshan Qiu, Michel W. Barsoum, and Yury Gogotsi, “Flexible and conductive MXene flms and nanocomposites with high capacitance,” Proc. Natl. Acad. Sci., 111, 47, 16676–16681, 2014. Doi:10.1073/pnas.1414215111. 97. Hyosung An, Touseef Habib, Smit Shah, Huili Gao, Miladin Radovic, Micah J. Green, and Jodie L. Lutkenhaus, “Surface-agnostic highly stretchable and bendable conductive MXene multilayers,” Sci. Adv., 4, 3, eaaq0118, 2018. Doi:10.1126/sciadv.aaq0118. 98. Seymur Cahangirov, Mehmet Topsakal, Aktürk Ethem, and Hasan Sahin, “Two- and one-dimensional honeycomb structures of silicon and germanium,” Phys. Rev. Lett., 102, 23, 236804, 2009. Doi:10.1103/PhysRevLett.102.236804. 99. Gexin Liu, X. L. Lei, Musheng Wu, and Bo Xu, “Is silicene stable in O2?—Firstprinciples study of O2 dissociation and O2-dissociation—induced oxygen atoms adsorption on free-standing silicene,” EPL (Europhysics Letters), 106, 4, 47001, 2014. Doi:10.1209/0295-5075/106/47001. 100. Gexin Liu, X. L. Lei, Musheng Wu, Bo Xu, and Chuying Ouyang, “Comparison of the stability of free-standing silicene and hydrogenated silicene in oxygen: a frst principles investigation,” J. Phys. Condens. Matter, 26, 35, 355007, 2014. Doi:10.1088/0953-8984/26/35/355007. 101. Hasan Sahin, S. Cahangirov, Mehmet Topsakal, E. Bekaroglu, Aktürk Ethem, Tuğrul Senger, and Salim Ciraci, “Monolayer honeycomb structures of group-IV elements and III-V binary compounds: frst-principles calculations,” Phys. Rev. B, 80, 15, 155453, 2009. Doi:10.1103/PhysRevB.80.155453. 102. Andres Castellanos-Gomez, Menno Poot, Albert Amor-Amorós, Gary A. Steele, Herre S.J. van der Zant, Nicolás Agraït, and Gabino Rubio-Bollinger, “Mechanical properties of freely suspended atomically thin dielectric layers of mica,” Nano Res., 5, 8, 550–557, 2012. Doi:10.1007/s12274-012-0240-3. 103. Pascal Pochet, Brian C. McGuigan, Johann Coraux, and Harley T. Johnson, “Toward Moiré engineering in 2D materials via dislocation theory,” Appl. Mater. Today, 9, 240– 250, 2017. Doi:10.1016/j.apmt.2017.07.007.

8

2D Materials for Energy Harvesting Astakala Anil Kumar, Sharmila Kumari Arodhiya, Shashank Priya, Ashok Kumar, and Shyam Sundar Pattnaik

CONTENTS 8.1 8.2

Introduction ...................................................................................................145 Emerging Applications of Two-Dimensional (2D) Materials........................147 8.2.1 Perovskite Solar Cells (PSCs)............................................................147 8.2.1.1 Graphene-Related Materials ...............................................148 8.2.1.2 Dichalcogenides..................................................................150 8.2.2 Thermoelectric Materials ..................................................................152 8.2.2.1 Oxide Thermoelectrics .......................................................152 8.2.2.2 Two-Dimensional Chalcogenides .......................................154 8.2.2.3 Polymer Layered Thermoelectric .......................................154 8.2.3 Other Applications.............................................................................155 8.2.3.1 Dye-Sensitized Solar Cells .................................................155 8.2.3.2 Piezoelectric Applications ..................................................156 8.2.4 Emerging 2D Layered Materials for Energy Harvesting ..................157 8.3 Conclusions and Future Prospectus...............................................................158 References...............................................................................................................158

8.1

INTRODUCTION

The global energy demand is increasing progressively due to the rapid industrialization and population growth. Besides, the energy-harvesting devices operating on the fossil fuels/ nonrenewable energy sources cause adverse effects to the environment. Consequently, the demand for the clean energy technologies operating on the renewable energies, like solar, wind, and thermal, are gaining importance. Though the existing energy harvesting systems have been effciently employed for energy harvesting, they are not yet competitive or on par with fossil fuels mainly due to the high production costs [1, 2]. It is an important task to develop inexpensive energy harvesting devices possessing improved effciency to cater the societal energy needs. The layered two-dimensional materials such as graphene and other transition metal chalcogenides possess non -centrosymmetric crystal structures. They have extensively been used in the fabrication of electromechanical systems and energy harvesting devices. The perovskite solar cells centered on mixed metal halide perovskite materials attracted attention due to their bandgap tunability, high absorption coeffcient DOI: 10.1201/9781003247890-8

145

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(~105 cm−1), and large diffusion length [3]. The 2D materials such as graphene, transition metal dichalcogenides, nitrides, MXenes, and Xenes revealed application in the fabrication of various energy harvesting devices such as thermoelectric, perovskite solar cells, and piezoelectric systems. Since the frst report published on the usage of organo-inorganic perovskites in 2009, nowadays the perovskite solar cells reached a stable power conversion effciency of about 24% [4]. In the recent past, there are more reports on the combination of the emerging twodimensional materials like graphene, transition metal dichalcogenides, and other emerging materials with the hybrid perovskites [5, 6]. The two-dimensional materials possess outstanding physical, chemical, and electronic properties that enhance the overall performance of the perovskite cell. The 2D materials are comprised of covalently bonded thin sheets, which can be molded into detached monolayers of desired thickness [7]. Among 2D materials, graphene is the earliest studied 2D material. It consists of carbon located in the symmetrical hexagons with sp2 hybridization. It has two-dimensional honeycomb structure and available physically in the form of fakes of few layers of carbon atoms [8, 9]. The two-dimensional graphene that possesses extraordinary properties—such as high electrical conductivity, electron mobility, bandgap tunability, stability, high thermal stability, high mechanical strength—makes it a viable candidate for the energy-harvesting applications [10–12]. The various applications of the 2D materials are shown in Figure 8.1.

FIGURE 8.1 Various applications of 2D materials.

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Recently, signifcant research is being done on the development of various materials for the energy-harvesting applications. Various 2D materials in general attracted a huge consideration in the fabrication process due to their unique electronic, mechanical, and optical properties [13]. The transition metal dichalcogenides are the most researched 2D materials after graphene in the energy-harvesting applications [14]. The transition metal dichalcogenides have general formula of MX2, where M is a transitional metal ion, such as Mo, W, Ti, etc., and X is a chalcogenide atom, such as sulfur (S), selenium (Se), or tellurium (Te). The transition metal chalcogenides possess layered structure and are fexible as compared to graphene/ graphenebased derivatives [15, 16]. The transition metal chalcogenides such as MoS2, MoSe2, WS2, WSe2 have been investigated extensively for the energy-harvesting applications [17–19]. The bandgap tunability, high surface area, fexibility are the attributes of transition metal chalcogenides, which makes these materials interesting for the carrier transport applications in energy-harvesting applications [20]. In the current chapter, the recent advances in the 2D layered materials for their application in the energy-harvesting devices such as perovskite solar cells, thermoelectric, and piezoelectric systems will be discussed.

8.2

EMERGING APPLICATIONS OF TWODIMENSIONAL (2D) MATERIALS

8.2.1 PEROVSKITE SOLAR CELLS (PSCS) Perovskite solar cells (PSCs) are the third-generation solar cells. These are a viable alternative to the existing silicon solar cells. Unlike silicon solar cells, the PSCs have different materials for different functionalities, such as absorption, separation, and collection of charge carriers. The organo-inorganic halide perovskite such as MAPbX3, FAPbX3 (where Methylammonium, MA+, formamidinium, FA+, and X = Cl, Br and I) act as a light absorber where the separation of electron and holes will initiate. The replacement of Pb with materials such as Sn, Ge, or other divalent materials is gaining importance due to toxicity of lead [21, 22]. The perovskite structured materials such as MAPbI3 possess unique properties, such as lower binding energy, higher absorption coeffcient of the order 104 cm−1, and higher value of dielectric constant [23]. Once the perovskite solar cell is illuminated, the perovskite layer deposited on the surface of the electron transport layer will absorb the sunlight and generate the electron hole pairs [24, 25]. The carrier diffusion length of 100 nm makes the MAPbI3 as the most researched material that has shown the higher power conversion effciencies [26]. The schematic representation of the different layers in perovskite solar cell is represented in the Figure 8.2 Typically, the perovskite solar cell consists of the mesoporous TiO2 layer as an electron transport layer, which is deposited on the FTO (fuorine doped tin oxide) substrate, Spiro- OMeTAD acts as an hole transport layer, and MAPbI3 functions as a light absorber. The electron hole pairs are separated, and the electrons are collected at the electron transport layer, and the holes are collected at the metal contacts (usually Au or Ag) deposited on the hole transport layer [27–29]. The potential drop across the different layers affects the performance of perovskite solar cell. Incorporation

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FIGURE 8.2 Schematic diagram of perovskite solar cell.

of the interface/buffer layers in between the electron transport layers of perovskite solar cell results in the enhancement of device performance. These interface/buffer layers tune the properties of the device without affecting the material choice [30– 32]. The various methodologies, such as self-assembling layers, inorganic materials, polymers, and the deposition of thin flms using several nanomaterials, are used to engineer the performance of PSC [33, 34]. According to the recent scientifc reports, the usage of two-dimensional materials such as graphene derivatives and transitional metal dichalcogenides as the buffer layer in the PSC has gained prominence. The unique physical and chemical properties along with the optoelectronic tunability made these two-dimensional materials unique as compared to the other materials. The synthesis/deposition of these materials is facile and easily processable. The solar cell parameters such as incident photon to current effciency (IPCE) and open circuit voltage (Voc) increase considerably with the incorporation of buffer/interfacial layers in between the adjacent layers of perovskite solar cells. 8.2.1.1 Graphene-Related Materials Among two-dimensional materials, graphene is the novel two-dimensional carbon derivative. It possesses good optical transparency and higher electron mobility, which makes it of enormous interest among scientifc community [35–36]. In the recent past, owing to the interesting physical and chemical properties, graphene and its derivatives have been extensively utilized in the perovskite solar cells. The graphene can be utilized in the device fabrication in different forms, such as zero, one, two, and three dimensions as graphene quantum dots, graphene nanobelt, graphene sheet, and grapheme, respectively, as per requirements in the perovskite solar cells. In addition, graphene can also be ftted/ decorated with the other inorganic materials to enhance the optoelectronic properties [37]. In literature, it was reported that the inducement of graphene in the PSC as a buffer layer in between the adjacent layers of

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perovskite solar cell results in the enhancement of the performance of the perovskite solar cells [38]. Mariani et al. [39] reported the fabrication of PSC utilizing the carbon-based counter electrodes and studied the variation of parameters with the addition of graphene between the interface of HTM and electron transport layer. The perovskite solar cell was fabricated with the TiO2 as electron transport layer, the MAPbI3 as light absorber, and Spiro-OMeTAD as hole transport layer. The counter electrode was made with the conducting carbon paste unlike the metal contact used in the traditional perovskite solar cells. The highest power conversion effciency (PCE) of 15.8% was achieved for the small area of 0.09 cm2, and PCE of 14.1% is achieved for the large area of 1 cm2. Further, the PCE of 13.9% was obtained for the device manufactured with the area of 4 cm2. The PCE obtained is the highest reported with the p-i-n confguration without using the conventional metal surfaces as counter electrode. Mahmoudi et al. [40] utilized the p-type inorganic hole transport material (NiO) in contrast to the conventionally used Spiro-OMeTAD. The usage of inorganic materials in PSC results in the enhancement of stability as the inorganic compounds are more stable than the organic materials. Further, the perovskite solar cell was fabricated using the MAPbI3−xClx/NiO-graphene as photo absorber, hole transport material (HTM), and the NiO as the buffer layer in between the inorganic and organic layer. The device with the NiO interface depicts the highest power conversion effciency of 20.8%. Interestingly, in contrast to the conventional PSC with organic functional layers, the device with inorganic hole transport that is kept in the ambience for a duration of 310 days shows the decrement in PCE of about 3% (conventional PSC 2%). Chen et al. [41] reported the PSC with the α-Fe2O3 as electron transport layer. Although α-Fe2O3 possesses better stability as compared to the conventionally used TiO2, its poor interaction with the perovskite results in the lower effciency of the device fabricated with α-Fe2O3 as electron transport layer. Further, with the inducement of N, S co-doped graphene quantum dots based buffer layer in between the electron transport layer (ETL) and perovskite layer results in the enhancement of power conversion effciency from 14 to 19.2%, which is the highest PCE reported with α-Fe2O3. Besides, the interfacial engineering adopted with the incorporation of buffer layer in between the adjacent layers of the perovskite solar cell results in the enhancement of stability and durability of device as compared to the conventional device fabricated with TiO2 as ETL. The mechanism/working of perovskite solar cell is shown in Figure 8.3. Ahmed et al. [42] fabricated PSC with ZnO as electron transport layer. The ZnO is a viable alternative to the conventionally used TiO2 as it has better electrical and optical conductivity. The presence of oxygen traps on the surface of the ZnO results in the decomposition of MAPbI3 to MAI and PbI2. The decomposition of the perovskite is minimized by using the graphene quantum dots buffer layer between ETL and perovskite layer. The inclusion of the buffer layer enhances the electron conductivity between the adjacent layers, which results in the enhancement of the power conversion effciency of the device. The device fabricated with ZnO graphene quantum dots based ETL shows an enhanced power PCE of 17%. Chandrasekhar et al. [43] fabricated the PSC with nitrogen doped graphene and ZnO nanocomposite-based electron transport layer. The perovskite solar cell was fabricated on the FTO substrate

150

FIGURE 8.3

Emerging 2D Materials and Applications

Working of a perovskite solar cell.

with nitrogen doped graphene and ZnO nanocomposite as electron transport layer, MAPbI3 as light absorber, Spiro-MEOTAD as HTM, and Ag contact on the rear end acts as counter electrode. Several PSCs were fabricated with varying the concentration of nitrogen doped graphene in the nanocomposite mixture. The perovskite solar cell fabricated with the optimized concentration of 0.8 wt% of nitrogen doped graphene showed the highest power conversion effciency of 16.82%. Guo et al. [44] synthesized Ni doped graphene nanocomposites using novel liquid phase impregnation method. The novel HTM-free perovskite solar cell was fabricated using the synthesized Ni doped graphene nanocomposites as electron transport layer, followed by the deposition of MAPbI3 layer that is used as an optical window. The carbon electrode coated on the top of the light absorber acts as a counter electrode. The optimized perovskite solar cell with the concentration of Ni at 4 × 10− 4 mol/g yields the highest power conversion effciency of about 12.39%. The device may be further optimized to engineer the performance of the HTM-free perovskite solar cell. 8.2.1.2 Dichalcogenides In perovskite solar cells, the ETL plays a crucial role in determining the device performance as it acts as a bridge for the photogenerated electrons. In conventional perovskite solar cells, the mesoporous TiO2 was utilized as ETL and was known for better power conversion effciencies. The presence of cation/oxygen vacancies on the surface of the metal oxide based ETL led the researchers to invade the other materials for the enhancement of device performance. The transition metal dichalcogenides will serve as an alternative for the traditionally used oxide semiconductor to conquer the role of an ETL in the perovskite solar cell. The transition metal halides possess the general formula MX2, where the cation M is a transition metal ion, and X belongs to the chalcogenide family. The layered transition

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metal dichalcogenides are the viable candidates to graphene as they have similar properties, such as fexibility, optical transparency, and the dimensions are also as comparable to graphene. The absence of the surface lattice defect and atomistic thickness makes the transition metal dichalcogenides a viable candidate as an ETL in perovskite solar cells. Among the transition metal dichalcogenides, the MoS2, WS2, MoSe2, and WSe2 are the materials explored for the usage of carrier transport in the perovskite solar cell as these possess high electron mobility, layered structures, optical transparency, tunability of bandgap with the incorporation of external dopants in the lattice, and involve the facile synthesis process. Particularly, the MoS2 has become more prominent in perovskite solar cells as it can have dual usage in the perovskite solar cell as both ETL and HTL due to its ambipolar properties. Malek et al. [45] successfully synthesized MOS2 nanosheets by ultrasonic spray pyrolysis method. The perovskite solar cell was fabricated with the synthesized MoS2 nanosheets. The optical window layer was composed of the triple cations namely Cs, MA, and FA Cs0.05 [MA0.13 FA0.87 ]0.95Pb(I0.87Br 0.13)3 with Spiro-OMeTAD as hole transport layer and Au contacts on the rear end of the device as the counter electrode. The electron transport layer was fabricated using different substrate temperatures ranging from 180 to 250°C. The device fabricated with the sample sintered at 220°C provided the highest power conversion effciency of 3.36%. The performance of the device can be engineered by modifying the physical, chemical, and morphological properties of the electron transport layer. Mahmood et al. [46] synthesized the MoS2 nanosheets by facile and cost-effective hydrothermal method. The electron transport layer was fabricated by electro spraying the synthesized MoS2 nanosheets on to the surface of FTO substrates, followed by the perovskite MAPbI3 as light absorber, Spiro-OMeTAD as hole transport layer, and Au contact on the rear side of perovskite solar cells as the counter electrode. The highest power conversion effciency of 16.17% was obtained with the electro spraying of MoS2 on the FTO substrate, which is higher as compared to the spin coated electron transport layer. The fabrication perovskite solar cell using the molybdenum selenide as hole transport layer is reported by Chen et al. [47]. The inverted planar structured perovskite solar cell is fabricated with the deposition of MoSe2 on the FTO substrate as hole transport layer, followed by the deposition of perovskite structured MAPbI3 as light absorber, fullerene (C60) as electron transport layer. The buffer layer of bathocuproine was introduced in between the electron transport layer and the counter electrode (Au) on the other end of the perovskite solar cell. On illumination with the 1.5 AM the optimized effciency of 8.23% was obtained for the various domain sizes of MAPbI3 in the range of 23–27 nm. Similarly, Chang et al. [48] reported the similar perovskite structure with MoSe2 as hole transport layer, MAPbI3 as light absorber, polymethyl methacrylate (PMMA) as electron transport layer, and Al doped ZnO as transparent conductive oxide, and the C60 as the buffer layer between the electron transport layer and the Al doped ZnO. The counter electrode was developed on the surface of Al doped ZnO with silver nanostructures. The unique modifed PMMA ETM flms have been prepared and characterized at various thicknesses of about 25 nm, where the highest power conversion effciency of 16.9% is reported, which is the highest reported effciency with the confguration and p-type MoSe2 as hole transport

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layer. The device may be engineered further to enhance the performance of the fabricated perovskite solar cells.

8.2.2 THERMOELECTRIC MATERIALS The thermoelectric generators convert heat energy directly into electrical energy without the emission of harmful gases and with noiseless operation because of the absence of any moving part. The thermoelectric materials are widely used in power generator, cooling devices, and sensors [49–52]. Thermoelectric generator can be the best solution for the utilization of waste heat generated from industries, vehicles, and also from human bodies. But low conversion effciency (peak effciency of 9.6% and system effciency of 7.4%) is the main disadvantage of thermoelectric generator [53]. In the past years, it was very challenging to decouple the thermoelectric parameters to enhance the effciency. The nano-structuring technique decouples the thermal conductivity and electrical conductivity due to quantum confnement. The working of the typical thermoelectric device is shown in the Figure 8.4 8.2.2.1 Oxide Thermoelectrics A thermoelectric material for large-scale application should not only possess high fgure of merit (ZT) but also it should have high abundance and easy availability. Therefore, the use of oxides as thermoelectric material can be promising.

FIGURE 8.4

Working of a thermoelectric device.

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Furthermore, oxides possess more chemical and thermal stability in comparison to other thermoelectric materials [54–56]. NaCo2O4 is quite prominent as a high temperature thermoelectric material. Several papers have reported the thermoelectric performance of NaCo2O4. Terasaki et al. [57] reported high thermoelectric power in single crystal of NaCo2O4. Fuzita et al. [58] reported ZT greater than one at 800 K and polycrystalline of NaCo2O4 showed ZT~0.8 at 1,000 K [59]. Flexible Na1.4Co2O4 thin flm prepared by self-fux method and painted on a printing paper showed a power factor of 159–223 µWm−1  K−2 in the temperature range of 303–522 K, which is comparable to conductive polymers [60]. Ca2Co2O5 single crystalline whisker showed the fgure of merit in the range of 1.2–2.7 at ≥873 K [61]. M. Ardyanian et al. prepared ZnO-SnO binary thin flm on glass substrate by using spray pyrolysis method and reported a maximum power factor of 4.06 × 10−5 Wm−1 K−2 for the molar ratio [Sn] / [Zn] = 10% at 490 K [62]. The Zn0.98Al0.02O was reported as a promising thermoelectric material because of its high electrical conductivity, Seebeck coeffcient, and melting point. It showed a power factor of 1.5 × 10−3 Wm−2 at 975 K [63–65]. Besides, the energy fltering process increases the carrier mobility. A major increment in electrical conductivity of singlelayered ZnO:Mg and bilayer ZnO:Mg/ZnO was reported due energy fltering. It led to a much-improved Seebeck coeffcient and hence power factor in comparison to ZnO [66]. Further, the ABO3 type perovskites are the effcient thermoelectric oxide materials. Their thermoelectric performance can further be improved by two-dimensional electron gas techniques. A superlattice composed of undoped SrTiO3 and Nb doped SrTiO3 on LaAlO3 single crystal substrate was fabricated by pulse laser deposition method, which attained a ZT of ⁓2.4, considering the thermal conductivity similar to those of bulk single crystal [67–69]. Table 8.1 shows the thermoelectric properties of CaCaO based oxides.

TABLE 8.1 Thermoelectric Properties of Ca-Co-O Ceramics Composition and structure Ca9Co12O28 Ca2Co2O5 single crystalline whisker (Ca2CoO3)0.7CoO2 single crystal Bi- and Na- substituted Ca3Co4O9 Textured Ca2Co2O5 La-doped textured Ca3Co4O9 (Ca2.8Ag0.05Lu0.15) Co4O9 with nano-inclusion [(Ca, Yb)2CoO3]0.62[CoO2] Ca3Co4O9+δ Ga-doped Ca3Co4O9+δ La, Fe-doped Ca3Co4O9 Ba-doped [Ca2CoO3]0.62[CoO2]

ZT

T (K)

S(μV/K)

0.047 1.2–2.7 0.87 0.32 0.26 0.6

700 ≥873 973 1000 973 975 1100

118 200 240 ~200 178 150 230

0.45 0.37 0.25 0.32 0.33

1000 800 1000 1000 1000

240 ~165 170 225 205

σ (S/cm)

κ (W/mK)

Ref.

84

1.73 ≈1 3 1.7 2.06 1.4

[70] [61] [71] [72] [73] [74] [75]

0.6 1.25 1.5 1.3 1.65

[76] [77] [78] [79] [80]

430 ~130 ~100 213 140 45 125 80 120

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8.2.2.2 Two-Dimensional Chalcogenides In the past, because of its reasonable price and performance, the only thermoelectric material used for commercial purpose was Bi2Te3 with ZT 1.35 and 0.9 for n- and p-type, respectively [81–82]. Despite the low conductivity of n-type Bi2Te3, it has been widely used in thermoelectric refrigerator in the temperature range of 180–450. For higher temperature power generation applications, PbTe and SiGe (ZT ⁓1) have been used [83]. The high temperature thermoelectric materials are widely used in space craft power production. Chalcogenides used as thermoelectric materials are divided into three categories depending on their working temperature range. For a temperature less than 150°C Bi2Te3, for 150–500°C TAGS [(AgSbTe2)1−x(GeTe)x], and above 500°C SiGe based materials are used [84, 85]. Figure 8.4 shows the basic construction of a single couple thermoelectric module consisting p- and n-type legs. Lagraneur et al. [86] fabricated a thermoelectric generator with three phases involving different thermoelectric material for different temperature range. For less than 250°C, n and p-type Bi2Te3, for 250–500°C TAGS and PbTe, for 500–700°C skutterudite p-type CeFe3RuSb and n-type CoSb3. These materials were chosen because of their highest fgure of merit in the corresponding temperature range. A fat thermocouple solution was used along with these segmented thermoelectric materials between the heat source and sink to increase the possibility of achieving the thickness and coeffcient of thermal expansion for various regions. Wilbrecht et al. [87] fabricated a two-phase TE generator using Bi2Te3 for low temperature and Mg2SiSn/ MnSi for moderate temperature. It achieved 44.2% higher effciency but 2.5 kW less electrical power than single phase generator (3.2 kW). The transition metal chalcogenides (TMDs), like MX2 and Cdl2 (M = Mo, W and X = S, Se, Te), are the layered materials, and each layer is held by van der Waals interaction. These exhibit high Seebeck coeffcient and low thermal conductivity. Cdl2 type 2D TMDs such as TiX2, ZrX2, HfX2 possess lower thermal conductivity in comparison to MX2 type TMDs [88]. The thermal conductivity for ZrSe2 is 1.2 Wm−1 K−1, and for HfSe2 it’s 1.8 Wm−1 K−1, which are less than Mo based TMDs and comparable to bulk PbTe and Bi2Te3 [89–93]. The indirect band structure in multilayer converts into direct bandgap in monolayer hence leads to variation in electronic properties. Bilayer 2D MoS2 exhibits a maximum power factor of 8.5 mWm−1 K−2 at room temperature. The improved transport properties TMDs are because of their valley degeneracies and large effective mass, which is further modifed by quantum confnement in 2D material [88, 94–96]. 8.2.2.3 Polymer Layered Thermoelectric Conductive polymers, because of their low thermal conductivity, higher availability, and easy synthesis process, are used to fabricate the thermoelectric devices for room temperature applications. Though comparative low electrical conductivity and Seebeck coeffcient limit their use in thermoelectric applications [82, 97], these are highly fexible to be used in wearable thermoelectric devices (WTD). By using WTD, the heat released by human body can be directly converted into electricity and be used for small applications, such as wrist watch, mobile charging, etc. Furthermore, the thermoelectric properties of conductive polymers can be improved

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by doping, de-doping [98], solvent treatment [99–102], sulfuric acid, and crystallization [103]. Power factor of (polypyrrole) PPy/grapheme/polyaniline (PANI) was reported 52.5 µW/mK2, which is much higher than that of pristine PPy and PANI [104]. Poly(3,4-ethylenedioxythiophene) (PEDOT)consisting (poly-styrenesulfonate) (PSS) have been studied intensively due to its low-cost, high transparency, waterdispensability, and excellent process ability [105]. The electrical conductivity can further be improved by post-treatment of PEDOT: PSS thin flm with the co-solvent of water and an organic solvent such as ethanol, isopropyl alcohol, acetonitrile, acetone, and tetra-hydro furan. The increment in electrical conductivity depends upon ratio of water to organic solvent, temperature, and dielectric constant of the solvent [106]. The thin flm of ethylene glycol mixed PEDOT: PSS has a ZT of 0.28 and dimethyl sulfoxide (DMSO) mixed PEDOT: PSS of 0.42 at room temperature [107]. Furthermore, Huang et al. reported the power factor of PEDOT: PSS thin flm annealed at 220°C, which was 162.5 times greater than without annealed flms [108]. Chalcogens that show the highest fgure of merit can be mixed with PEDOT: PSS to produce highly effcient fexible thermoelectric material. Park et al. [109] fabricated a fexible thermoelectric device based on Ag2Se nano-wire/PEDOT: PSS nano-composite flm. For synthesizing this flm, frst the Se nano-wires coated with PEDOT: PSS were mixed with Ag precursor to prepare PEDOT: PSS coated Ag2Se nanowires, and in a separate beaker, PEDOT:PSS was mixed in dimethyl sulfoxide (DMSO). The Ag2Se nanowires coated with PEDOT: PSS were dispersed in the solution of PEDOT: PSS and DMSO. This fnal solution was than deposited on the glass substrate via drop cast method. The flm consisting of 50 wt% nanowires showed the highest power factor of 327.15 µW/mK2 at 300 K. The thermoelectric properties were decreased only by a factor of ⁓5.9% after bending of 1,000 times. The device fabricated using this flm generated output voltage of 7.6 mV for the temperature difference of 20 K.

8.2.3 OTHER APPLICATIONS 8.2.3.1 Dye-Sensitized Solar Cells Dye-sensitized solar cells (DSSC) are the third-generation solar cells that are extensively investigated due to their low operational cost and facile fabrication methodologies. Similar to the perovskite solar cell, the DSSC consists of the electron transport layer, light absorber, hole transport layer and the counter electrode to harvest the solar energy [110–112]. The conventional DSSC consists of TiO2, N719 dye, iodide/triiodide electrolyte and platinum as ETL, light absorber, HTM, and counter electrode, respectively [113]. Various components of the DSSC are shown in the Figure 8.5. The platinum that has been conventionally used as counter electrode material is to be replaced as the availability of platinum is scarce, which, in turn, increases the fabrication cost of the device. The two-dimensional materials such as graphene was effectively used as the counter electrode material in DSSC, which showed the power conversion effciency close to 13% [114]. The success of graphene as the counter electrode material in DSSC implicated the researchers to focus on the other two-dimensional materials, such as MoS2, WS2, MoSe2, WSe2 [115–117]. The power conversion

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FIGURE 8.5

Schematic diagram of various components of DSSC.

FIGURE 8.6

Working of a piezoelectric device.

effciencies and the stability of the DSSC reported with the two-dimensional material are better as compared to conventionally used platinum counter electrodes. Currently, researchers are focused on the long-term stability of transition metal chalcogenides based DSSC in different environmental conditions. 8.2.3.2 Piezoelectric Applications Piezoelectricity refers to the development of electrical charge on application of applied stress. In the absence of external strain, the charge distribution is symmetric, and the net electric dipole moment is zero. With the application of external stress, the charges are displaced, and the charge distribution is no longer symmetric. It gives rise to net polarization [118]. The schematic representation of the typical piezoelectric device is shown in the Figure 8.6

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The two-dimensional materials have been of greater interest among the scientifc community as these are known to have central symmetry and are predicted to have intrinsic piezoelectricity. The two-dimensional materials such as graphene/other transition metal dichalcogenides were extensively investigated due to their unique properties, such as intrinsic piezoelectricity, larger surface area, and facile synthesis process [119]. Compared to conventionally used ferroelectric materials, the twodimensional layered materials have the advantages such as bandgap, tunability, and fexibility [120].

8.2.4

EMERGING 2D LAYERED MATERIALS FOR ENERGY HARVESTING

Two-dimensional layered materials like graphene and graphene-based materials, transition metal dichalcogenides, transition metal carbide, hexagonal boron nitride, etc. are widely used in energy harvesting applications specifcally in piezoelectric and triboelectric devices due to their unique properties [121, 122]. Graphene has single-layer honeycomb lattice structure and zero bandgap (semiconductor) showing extraordinary chemical and physical properties. Hexa-boron nitride (h-BN) also has a structure like graphene but shows different properties (insulator). The 2D layered materials cover almost all types of electrical properties from insulator to superconductor. Graphene possesses a high value of electron mobility as well as thermal conductivity [123, 124]. Graphene is 100 times more conducting than Cu and has 100 times more electron mobility in comparison to single crystalline Si [122]. Graphite was used as an anode in lithium-ion batteries because of its large abundance, low-cost, and good theoretical capacity of 372 mAh/g. Han et al. found that graphene has much larger theoretical capacity (740mAh/g) than that of graphite [126]. TMDs also have larger theoretical capacity than that of graphite (670 mAh/g for MoS2) [127]. Hence several research papers have been reported on better performance of lithium ion and sodium ion batteries using graphene and TMDs as an anode [64]. Besides, graphene is used for H2storage and TMDs for generation of H2. Few layers of graphene can store ⁓ 5wt% of H2. Zhou et al. reported ⁓ 16 wt% storage capacity of graphene covered with Li ions [128]. Moreover, 2D layered materials have been used as supercapacitors and nanogenerators. Huang et al. [129] fabricated a supercapacitor based on TMDs with specifc capacitance of 576 F/g at 5 mV/s. Goe et al. [130] fabricated a supercapacitor based on h-BN with a specifc capacitance of 140 F/g at 2 A/g. A specifc capacitance of 310 F/g at 2mV/s was reported for graphene-MnO2 composite [131]. Metal organic frameworks (MOFs) are another option for energy storage and conversion such as photocatalytic hydrogen evolution, fuel cells, batteries, and supercapacitor due to their excellent gas storage capacity [132]. Further, due to nanotechnology and advanced characterization methods, new approaches are possible to rationally design and, hence, modify the material effciency [133–135]. MOFs are porous materials and can be used direct as well as a substrate to integrate functional metals, metal oxides, semiconductor, and complexes to generate nanostructure [136, 137]. In 2003, the discovery of MOF-5 with hydrogen storage capacity has attracted the attention of researchers as hydrogen and methane adsorbent [138, 139]. The hydrogen storage of MOF-210 with highest surface area with total H2gravimetric uptake

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is of 17.6 wt% at 80 bar and 77 K [140]. The absorption capacity of MOFs depends upon the volume of the pores. When benzene ring of NOTT-101 is replaced with pyrimidine ring of UTSA-76, an unexpected enhancement was observed in volumetric uptake [257 cm3 (STP) cm−3 (298 K and 65 bar)] and with working capacity of 197 cm3 (SPT) cm−3 [141]. MOF derivatives have been used as a photocatalyst in water splitting and carbon dioxide reduction. The products generated in CO2 reduction, like CO, HCOOH, MeOH, and CH4 are used as energy carriers for industrial applications [142, 143]. Fuel cells are very popular for energy storage and conversion technology as they convert fuel (e.g., hydrogen, natural gas, and methanol) into electricity. The MOFs, MOF composite and derivatives are used in fuel cells as an electrolyte and electrode catalyst. MOFs also used in oxygen reduction reaction, supercapacitor, LI, Li-O2, Li-S batteries, electrolysis of water, etc. [132]. Boron nitride (BN) is used as fllers in pyroelectric devices due to low-cost and non-toxicity to improve the thermal transfer. Q. Wang et al. reported 65.6% increment in harvested power of BN fled PMN-PMS-PZT in comparison to unfled PMNPMS-PZT [84]. Because of its wide bandgap (6.2 eV), high chemical and thermal stability h-BN layers are used in solar cells as interfacial layers [144].

8.3

CONCLUSIONS AND FUTURE PROSPECTUS

The research on the development of various 2D materials for the energy harvesting device applications is on the high appreciation in the recent years due to their unique layered structure and optoelectronic properties. The various 2D materials such as graphene, graphene-related materials, transition metal dichalcogenides, MXenes, Xenes were synthesized and used as an active material in the various energy harvesting devices such as perovskite solar cells, thermoelectric, and piezoelectric device applications. In continuation, most of the 2D materials were not investigated due to the diffculty in the material synthesis. The various parameters for the synthesis methods for the 2D materials required to be optimized. It was found that the thermal conductivity of the 2D materials possess inherent thermal conductivity, which is a limitation to the performance of the thermoelectric device. The research needs to be focused on the structural modifcation and to tune the thermal conductivity as the thin flms of various 2D materials have huge application in the various mechanical and optoelectronic systems. The deposition of uniform/layered thin flms with 2D material is still challenging. The synthesis of 2D materials with better yield using the facile synthesis routes is the current active challenge among the scientifc community. The several key phenomenon changes in the 2D materials due to the application of the strain, temperature gradient, and the photoinduced changes are to be investigated. Besides, the infuence of the external parameters on the band structure and the bandgap tunability need to be investigated.

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89. Huang, H. and Fan, X. Research advances of typical two dimensional layered thermoelectric materials. Res Appl Mater Sci, 2, 1–11 (2020). 90. Kumar, S. and Schwingenschlogl, U. Thermoelectric response of bulk and monolayer MoSe2 and WSe2. Chem Mater, 27, 1278–1284 (2015). 91. Jin, Z., Liao, Q., Fang, H., Liu, Z., Liu, W., Ding, Z., Luo, T. and Yang, N. A revisit to high thermoelectric performance of single-layer MoS 2. Sci Rep, 5, 18342 (2015). 92. Sahoo, S., Gaur, A. P., Ahmadi, M., Guinel, M. J.-F. and Katiyar, R. S. Temperaturedependent Raman studies and thermal conductivity of few-layer MoS2. J Phys Chem C,117, 9042–9047 (2013). 93. Hellman, O. and Broido, D. A. Phonon thermal transport in Bi2Te3 from frst principles. Phys Rev B, 90, 134309 (2014). 94. Qiu, B., Bao, H., Ruan, X., Zhang, G. and Wu, Y. In molecular dynamics simulations of lattice thermal conductivity and spectral phonon mean free path of PbTe: Bulk and nanostructures, Comput. Mater Sci, 278–285 (2012). 95. Hippalgaonkar, K., Wang, Y., Ye, Y., Qiu, D. Y., Zhu, H., Wang, Y., Moore, J., Louie, S. G. and Zhang, X. High thermoelectric power factor in two-dimensional crystals of MoS2. Phys Rev B, 95, 115407 (2017). 96. Wickramaratne, D., Zahid, F. and Lake, R. K. Electronic and thermoelectric properties of few-layer transition metal dichalcogenides. J Chem Phys140, 124710 (2014). 97. Huang, W., Luo, X., Gan, C. K., Quek, S. Y. and Liang, G. Theoretical study of thermoelectric properties of few-layer MoS2 and WSe2. Phys Chem Chem Phys, 16, 10866– 10874 (2014). 98. Park, D., Kim, M. and Kim, J. Conductive PEDOT: PSS-based organic/inorganic fexible thermoelectric flms and power generators. Polymers, 13, 210 (2021). 99. X. Li, C. Liu, W. Zhou et al. Roles of polyethylenimineethoxylated in effciently tuning the thermoelectric performance of poly(3,4-ethylenedioxythiophene)-rich nanocrystal flms, ACS Appl Mater Interfaces, 11(8), 8138–8147 (2019). 100. Zhang, S., Fan, Z., Wang, X. and Zhang, Z. Enhancement of the thermoelectric properties of PEDOT: PSSviaone-step treatment with cosolvents or their solutions of organic salts. J Mater Chem A, 6(16), 7080–7087 (2018). 101. Lee, S. H., Park, H., Kim, S., Son, W., Cheong, I. W. and Kim, J. H. Transparent and fexible organic semiconductor nanoflms with enhanced thermoelectric effciency. J Mater Chem A, 2(20), 7288–7294 (2014). 102. Zhao J. and Tan, D. A strategy to improve the thermoelectric performance of conducting polymer nanostructures. J Mater Chem C, 5(1), 47–53 (2017). 103. Xie, J., Zhao, C., Lin, Z. and Gu, P. Nanostructured conjugated polymers for energyrelated applications beyond solar cells. Chem Asian J, 11(10), 1489–1511 (2016). 104. Kim, N., Kee, S., Lee S. H. et al. Highly conductive PEDOT: PSS nanofbrils induced by solution-processed crystallization. Adv Mater, 26(14), 2268–2272 (2014). 105. Wang, Y., Yang, J., Wang, L., Du, K., Yin, Q. and Yin, Q. Polypyrrole/graphene/polyaniline ternary nanocomposite with high thermoelectric power factor. ACS Appl Mater. Interfaces, 9, 20124–20131 (2017). 106. Fei-Peng Du, Nan-Nan Cao, Yun-Fei Zhang, Ping Fu, Yan-Guang Wu, Zhi-Dong Lin, Run Shi, Abbas Amini and Chun Cheng. PEDOT: PSS/graphene quantum dots flms with enhanced thermoelectric properties via strong interfacial interaction and phase separation, Sci Rep, 8(1–12), 6441 (2018). 107. Yijie Xia and Jianyong Ouyang. PEDOT: PSS flms with signifcantly enhanced conductivities induced by preferential solvation with cosolvents and their application in polymer photovoltaic cells. J Mater Chem, 21, 4927–4936 (2011). 108. Kim, G-H., Shao, L., Zhang, K. and Pipe, K. P. Engineered doping of organic semiconductors for enhanced thermoelectric effciency. Nat Mater, 12, 729–723 (2013).

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109. Huang, X., Deng, L., Liu, F., Zhang Q. and Chen, G. Effect of crystalline microstructure evolution on thermoelectric performance of PEDOT: PSS flms. Energy Mater Adv, 2021, 10 (2021). 110. Aftabuzzaman, M., Lu, C. and Kim, H.K. Recent progress on nanostructured carbonbased counter/back electrodes for high-performance dye-sensitized and perovskite solar cells. Nanoscale, 12(34), 17590–17648 (2020). 111. Joseph, I., Louis, H., Unimuke, T.O., Etim, I.S., Orosun, M.M. and Odey, J. An overview of the operational principles, light harvesting and trapping technologies, and recent advances of the dye sensitized solar cells. Appl Solar Energy, 56(5), 334–363 (2020). 112. Bera, S., Sengupta, D., Roy, S. and Mukherjee, K. Research on dye sensitized solar cell: A review highlighting the progress in India. J Phys Energy 3, 032013 (2021). 113. Ling, C.K., Aung, M.M., Abdullah, L.C., Ngee, L.H. and Uyama, H. A short review of iodide salt usage and properties in dye sensitized solar cell application: Single vs binary salt system. Solar Energy, 206, 1033–1038 (2020). 114. Muchuweni, E., Martincigh, B.S. and Nyamori, V.O. Recent advances in graphenebased materials for dye-sensitized solar cell fabrication. RSC Advances, 10(72), 44453– 44469 (2020). 115. Tapa, A.R., Xiang, W. and Zhao, X., Metal chalcogenides (M x E y; E= S, Se, and Te) as counter electrodes for dye—Sensitized solar cells: An overview and guidelines. Adv Energy Sustainab Res, 2100056. 116. Dey, S. Atomically controlled two-dimensional heterostructures: Synthesis, characterization and applications. In Functional Properties of Advanced Engineering Materials and Biomolecules (pp. 201–235). Springer, Cham, 2021. 117. Fatima, N., Tahir, M.B., Noor, A., Sagir, M., Tahir, M.S., Alrobei, H., Fatima, U., Shahzad, K., Ali, A.M. and Muhammad, S. Infuence of van der waals heterostructures of 2D materials on catalytic performance of ZnO and its applications in energy: A review. I J Hydr Energy 46, 25413–25423 (2021). 118. Goel, S. and Kumar, B. A review on piezo-/ferro-electric properties of morphologically diverse ZnO nanostructures. J Alloys Compd, 816, 152491 (2020). 119. Liang, Q., Zhang, Q., Zhao, X., Liu, M. and Wee, A.T. Defect engineering of twodimensional transition-metal dichalcogenides: Applications, challenges, and opportunities. ACS Nano, 15(2), 2165–2181 (2021). 120. Miao, J. and Wang, C., Avalanche photodetectors based on two-dimensional layered materials. Nano Res, 14(6), 1878–1888 (2021). 121. Park, D., Kim M. and Kim, J. Conductive PEDOT: PSS-based organic/inorganic fexible thermoelectric flms and power generators. Polymers, 13,210 (2021). 122. Yang, P-K. and Lee, C.-P. 2D-layered nanomaterials for energy harvesting and sensing applications, monolayers of emerging nanomaterials. Intech Open, 1–14 (2019). http:// dx.doi.org/10.5772/intechopen.85791 123. Han, S.A., Sohn, A. and Kim, S-W. Recent advanced in energy harvesting and storage applications with two-dimensional layered materials. FlatChem, 6, 37–47 (2017). 124. Bolotin, K. I., Sikes, K. J., Jiang, Z., Klima, M., Fudenberg, G., Hone, J., Kim P. and Stormer, H. L. Ultrahigh electron mobility in suspended graphene. Solid State Commun, 146, 351–355 (2008). 125. Balandin, A. A., Ghosh, S., Bao, W., Calizo, I., Teweldebrhan, D., Miao, F. and Lau, C. N. Superior thermal conductivity of single-layer graphene. Nano Lett, 8, 902–907 (2008). 126. Han, S., Wu, D. Q., Li, S., Zhang, F. and Feng. X. L. Graphene: a two-dimensional platform for lithium storage. Small, 9 (2013) 1173–1187. 127. Ding, S., Zhang, D., Chen, J. S. and Lou, X. W. Facile synthesis of hierarchical MoS2 microspheres composed of few-layered nanosheets and their lithium storage properties. Nanoscale, 4 (2012) 95–98.

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128. Zhou, W., Zhou, J., Shen, J., Ouyang, C. and Shi. S. First-principles study of highcapacity hydrogen storage on graphene with Li atoms. J Phys Chem Solids, 73 (2012) 245–251. 129. Huang, K.-J., Zhang, J.-Z., Shi, G.-W. and Liu, Y.-M. Hydrothermal synthesis of molybdenum disulfde nanosheets as supercapacitors electrode material. ElectrochimicaActa, 132 (2014) 397–403. 130. Gao, T., Gong, L., Wang, Z., Yang, Z., Pan, W., He, L., Zhang, J., Ou, E., Xiong, Y. and Xu, W. Boron nitride/reduced graphene oxide nanocomposites as supercapacitors electrodes. Mater Lett, 159 (2015) 54–57. 131. Yana, J., Fana, Z., Weia, T., Qianb, W., Zhanga, M. and Weib, F. Fast and reversible surface redox reaction of graphene–MnO2 composites as supercapacitor. Carbon, 48 (2010) 3825–3833. 132. Wang, H., Zhu, Q-L., Zou, R. and Xu, Q. Metal-organic frameworks for energy applications. Chem, 2, 50–80 (2017). 133. Zhang, Z., Chen, Y., Xu, X., Zhang, J., Xiang, G., He, G. and Wang, X. Well defned metal-organic framework hollow nanocages. Angew Chem Int Ed Engl, 53, 429–433 (2014). 134. Lu, G., Li, S., Guo, Z., Farha, O.K., Hauser, B.G., Qi, X., Wang, Y., Wang, X., Han, S., Liu, X. et al. Imparting functionality to a metal-organic framework material by controlled nanoparticle encapsulation. Nat Chem, 4, 310–316 (2012). 135. Zhu, Q.-L. and Xu, Q. Metal-organic framework composites. Chem Soc Rev, 43, 5468– 5512 (2014). 136. Sun, J.-K. and Xu, Q. Functional materials derived from open framework templates/ precursors: Synthesis and applications. Energy Environ Sci, 7, 2071–2100 (2014). 137. Rosi, N.L., Eckert, J., Eddaoudi, M., Vodak, D.T., Kim, J., O’Keeffe, M. and Yaghi, O.M. Hydrogen storage in microporous metal-organic frameworks. Science, 300, 1127– 1129 (2003). 138. Schoedel, A., Ji, Z. and Yaghi, O.M. The role of metal-organic frameworks in a carbonneutral energy cycle. Nat Energy, 1, 16034–16046 (2016). 139. Furukawa, H., Ko, N., Go, Y.B., N. Aratani, S.B. Choi, E. Choi, A.O. Yazaydin, R.Q. Snurr, M. O’Keeffe, J. Kim and O.M. Yaghi. Ultrahigh porosity in metal-organic frameworks. Science, 329, 424–428 (2010). 140. Li, B., Wen, H.-M., Wang, H., Wu, H., Tyagi, M., Yildirim, T., Zhou, W. and Chen, B. A porous metal-organic framework with dynamic pyrimidine groups exhibiting record high methane storage working capacity. J Am Chem Soc, 136, 6207–6210 (2014). 141. Dhakshinamoorthy, A., Asiri, A.M. and Garc, H. Metal-organic framework (MOF) compounds: Photocatalysts for redox reactions and solar fuel production. Angew Chem Int Ed Engl, 55, 5414–5445 (2016). 142. Kornienko, N., Zhao, Y., Kley, C.S., Zhu, C., Kim, D., Lin, S., Chang, C.J., Yaghi, O.M. and Yang, P. Metal-organic frameworks for electrocatalytic reduction of carbon dioxide. J Am Chem Soc, 137, 14129–14135 (2015). 143. Wanga, Q., Bowenc, C. R., Lewisc, R., Chend, J., Leia, W., Zhange, H., Lia, M-Y. and Jianga, S. Hexagonal boron nitride nanosheets doped pyroelectric ceramic composite for high-performance thermal energy harvesting, Nano Energy, 60, 144–152, 2019. 144. Miyata, N., Moriki, K., Mishima, O., Fujisawa, M. and Hattori, T. Optical constants of cubic boron nitride. Phys Rev B, 40 12028–12029 (1989).

9

2D Nanomaterials for Energy Applications Sayli Pradhan and Neetu Jha

CONTENTS 9.1

Introduction ...................................................................................................167 9.1.1 Graphene............................................................................................167 9.1.2 Hexagonal Boron Nitride...................................................................168 9.1.3 Transition Metal Dichalcogenides (TMDCs) ....................................169 9.2.4 Phosphorene.......................................................................................170 9.2.5 MXenes..............................................................................................171 9.2 Properties of 2D Nanomaterials Compared to Bulk .....................................172 9.3 Energy Storage Applications of 2D Nanomaterials.......................................173 9.3.1 Li Ion Batteries ..................................................................................173 9.3.2 Zinc Based Batteries..........................................................................174 9.3.2(a) Zinc Ion Batteries...............................................................174 9.3.2(b) Zinc Air Batteries ..............................................................175 9.3.3 Sodium Ion Batteries .........................................................................176 9.3.4 Supercapacitors ..................................................................................179 MXenes..............................................................................................180 Transition Metal Dichalcogenides (TMDs) .......................................181 Black Phosphorus (BP) ......................................................................182 9.3.5 Hybrid Energy Storage (HES) Devices .............................................183 9.3.6 Fuel Cells ...........................................................................................183 9.4 Conclusion .....................................................................................................186 References...............................................................................................................186

9.1 9.1.1

INTRODUCTION GRAPHENE

Graphene is a one-atom-thick two-dimensional layered material with sp2- bonded carbon. It has become one of the most important carbon materials in the multiple areas like materials science and condensed matter physics.1,2 Motivated by its high theoretical specifc surface area (2,620 m2 g−1), high mechanical fexibility, ultrathinness, good electrical conductivity, and high theoretical capacitance (e.g., 550 F g−1), much attentions have been paid to the application of graphene for energy storage devices,3 in particular, high-energy batteries and high-power supercapacitors (SCs).4–9 At present, various methods have been developed to effectively synthesize graphene via top-down or bottom-up strategies. In general, top-down strategies are mainly based on the exfoliation DOI: 10.1201/9781003247890-9

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FIGURE 9.1 (Color online) Schematic of structural models of graphene/metal oxide hybrids. (a) Anchored model: oxide NPs anchored on graphene. (b) Wrapped model: oxide wrapped by graphene. (c) Encapsulated model: oxide encapsulated by graphene. (d) Sandwich-like model: an oxide/graphene/oxide sandwich-like structure. (e) Layered model: an alternating layered structure of oxide and graphene. (f) Mixed model: graphene and oxide mechanically mixed. Source: Reprinted with permission, copyright 2012, Elsevier5

of graphite,10 such as micromechanical cleavage, oxidation-exfoliation-reduction, intercalation exfoliation, and solid exfoliation (e.g., ball milling11). And bottom-up strategies include organic synthesis from small structurally-defned molecules (e.g., polyphenylene12) and chemical vapor deposition (CVD).13 Notably, the graphene products vary distinctly from different synthesis methods in terms of number of layers, sizes, functional groups, wrinkles, defects (in-plane holes or heteroatom doping), and shapes (e.g., graphene nanoribbons), which could be remarkably refected by huge differences in apparent properties (e.g., SSA, apparent density) and device performances. For instance, mechanically exfoliated graphene sheets (GSs) possess perfect crystallinity without abundant defects. It exhibits low sheet resistance (Rs) of around 400 Ωsq−1 at room temperature,14 much higher than that of reduced graphene oxide (rGO) sheets by wet chemical routes, in which many defects and oxygen containing functional groups are left in GS, resulting in low conductivities mainly ranging from 298 to 0.045 S m−1.15 Furthermore, the layers and defects of GSs could be precisely prepared by CVD method via adjusting growth parameter (e.g., temperature, time, catalyst); however, in the case of oxidation-exfoliation-reduction, the control of layers and defects seems to be impossible. Therefore, by using appropriately physical or chemical method, one can create desirable defects to adjust the band structure of graphene-based materials (e.g., porous graphene and doped graphene sheets) and functionalize graphene with organic/inorganic matters for performance enhancement.16

9.1.2

HEXAGONAL BORON NITRIDE

h-BN is a semiconducting material from group III-V compound consisting of layers of sp2- hybridized alternating boron and nitrogen atoms in a honeycomb lattice, with the layers coupled by van der Waals forces. Isostructural to graphene, h-BN nanosheets (h-BNNS) are also known as “white graphene.” Its layered structure, along with high

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FIGURE 9.2

169

An overview of hexagonal boron nitride structures.

Source: Figures reproduced with permission26

chemical and thermal stability, allows the applications as lubricants and protective coatings. Pristine h-BN has a wide bandgap energy of 5.9 eV17, enabling it to emit deep ultraviolet light at around 200 nm.18 Unlike graphene, which is gapless and has rich electronic properties, the wide bandgap renders h-BN an insulating property and consequently hinders its applications in the felds of energy conversion and storage. There have been a lot of research activities focusing on the modifcation of h-BN properties so that it can be used in various applications. Heteroatom doping and functional group grafting have been proved effective. Depending on the incorporated dopants and functional groups (H, F, OH, CH3, CHO, CN, NH2, etc.), the bandgap of h-BN can be theoretically varied from 0.3 eV to 3.1 eV.19 Experimentally doping h-BN with carbon can reduce its bandgap from 5.9 eV to 2.6 eV and therefore make it a promising nonmetal photocatalyst.20 Carbon and oxygen co-doping can modify h-BN’s optical, electronic, and surface properties, granting it interesting fuorescent, electrochemical energy storage and gas sorption capabilities.21–23 Edgehydroxylation, an effective functionalization method, which can be realized by the reaction between commercial h-BN and hot steam, gives the material enhanced catalytic oxidative dehydrogenation of alkane.24,25

9.1.3

TRANSITION METAL DICHALCOGENIDES (TMDCS)

TMD monolayers are thin 2D semiconductors in the form of MX2, where M is a transition metal from group 4–10 (e.g., Mo or W) and X is a chalcogen (e.g., S, Se, or Te).27

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FIGURE 9.3 Representative structures of selected oxide nanosheets whose bulk counterparts are naturally layered. (a) Ti0.91O20.36−, (b) MnO20.4−, (c) Nb61O174−, (d) TaO3–, (e) MoO3, (f) V2O5. (g) Schematic illustration of the exfoliation of layered KCa2Nb3O10 into Ca2Nb3O10 – nanosheets using the cation exchange-assisted liquid exfoliation method. Source: Reproduced with permission, copyright 2014, Springer Nature31

One transition metal atomic layer is sandwiched between two layers of chalcogen atoms. Covalent bonds exist in individual layers and van der Waals forces dominate between layers. TMD monolayers can provide large exposed surface and abundant surface sites for electrostatic adsorption or surface redox reactions. Besides, some TMDs have multiple phases with diverse electrical properties, ranging from insulating, semiconducting, to metallic.28 These unique properties have attracted interest over their use in capacitive energy storage applications. Among various TMD materials, MoS2 is currently the most popular electrode materials for SCs due to its high specifc capacitance (~1000 F g −1).29,30

9.2.4

PHOSPHORENE

Phosphorene is a monolayer or several-layered black phosphorus (BP), similar to graphene, which can be synthesized from bulk BP using many methods.32–34 BP is the most stable allotropy of phosphorus in ambient conditions. Under certain conditions, it can be mutually transformed from white phosphorus or red phosphorus. Bridgman accidentally obtained BP during an investigation of the effect of high pressure on the melting point of ordinary white phosphorus.35 The crystal structure of BP in normal conditions consists of puckered layers parallel to the ac plane.36,37 Some of the great advantages of BP are the direct bandgap tunability from 0.3 eV (in bulk) to 1.5 eV

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FIGURE 9.4 Black phosphorus structure. (a) 3D representation. (b) Lateral view. (c) Top view. Source: Reproduced from with permission from 2D materials32

(in monolayer),38–42 enhanced charge carrier mobility, and anisotropic in-plane properties. These properties make BP a special candidate for research associated with nanophotonic devices, incorporating 2D layered materials and moderate bandgapbased nano-electronic applications. Such a band structure flls a much needed gap for feld-effect transistor (FET) applications. BP and phosphorene were reported to have several more advantageous properties than other 2D materials in application as a negative material for batteries. In particular, black phosphorus has a folded structure, and each P atom reacts with three Li or Na atoms to form Li3P and Na3P, giving it a very high theoretical specifc capacity of 2596 mAh g−1,43 which is far more than the graphite negative electrode (372 mAh g−1) with its excellent energy storage properties. These outstanding performances have attracted many people to focus their attention on BP as a negative material for energy storage.44

9.2.5

MXENES

A large group of early transition metal carbides, nitride, or carbonitride (MXenes) have been identifed as a new class of two-dimensional (2D) materials.45 The general formula of MXene is Mn+1XnTx, where M stands for an early transition metal (Ti, Mo, Cr, Nb, V, Sc, Zr, Hf, or Ta), X represents carbon or nitrogen, n is usually an integral number between 1 and 3, and Tx is the surface termination, such as hydroxyl, oxygen, or fuorine. These surface terminations render hydrophilicity to MXenes and also have a signifcant infuence on their Fermi level density of the states, thereby electronic properties.46,47 MXenes are synthesized by selectively removing the “A” layer from their parent MAX phases, and “ene” was added at the last to show its 2D nature that is similar to graphene. Since its discovery in 2011,48 MXenes have been particularly attractive as electrode materials for energy storage applications because of their unique structures, including (1) the inner conductive transition metal carbide layer that enables effcient electron transportation and (2) a transitional metal oxidelike surface that acts as active sites for fast redox reactions. Although there are more than 20 different MXenes that have been synthesized,49–52 most research to date has

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FIGURE 9.5 MXenes precursors and their synthesis. (a) Three types of mono-M MAX phases M2AX, M3AX2, M4AX3 and the selective etching process of the A group layers (red atoms). (b) MXenes are formed after selective etching and formation of surface terminations (yellow atoms) labeled as T. (c) Possible elements for M, A, X, and T in MAX and MXene phases.

been focused on titanium carbide (Ti3C2) for MSC application due to its ultrahigh conductivity (2.4  ×  104 S cm−1)53 and volumetric capacitance (1500 F/cm3) along with ultrahigh rate capability (10 V/s) in acidic media.54 Besides their high volumetric capacitance, MXenes are particularly favorable for MSCs because of their 2D nature, which enhances the mechanical stability and shortens the ion diffusion paths between the positive and negative electrodes.55

9.2

PROPERTIES OF 2D NANOMATERIALS COMPARED TO BULK

Removal of van der Waals interactions—A layered bulk material consists of many covalently bonded planes held together by weak van der Waals interactions. When a force is applied to a material, these van der Waals forces can be easily overcome and the material breaks, making it seem weak. Conversely, the covalent bonds that hold the atoms together in the layers are actually very strong. A monolayer will only have covalent bonds. By removing the “weak links” from the material, it appears to become much stronger. For example, graphene has a tensile strength 1,000 times greater than graphite, and while a graphite pencil can be easily broken, graphene is over 100 times stronger than steel.

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An increase in the ratio of surface-area-to-volume—The surface-area-to-volume ratio of a material defnes how much of it is exposed to its environment. This is important for chemical reactions—the more reactant that is in contact with the material, the faster the reaction can occur, so 2D materials tend to be more reactive than their bulk counterparts. It also makes 2D materials more sensitive to their surroundings, an effect that is exploited for sensors based on 2D materials. Confnement of electrons in a plane—The electronic and optical properties of a material depend upon its electronic band structure. This describes how electrons move through the material and is a result of the periodicity of its crystal structure. When a material goes from bulk to 2D, the periodicity is removed in the direction perpendicular to the plane, which can greatly change the band structure. The modifed band structures are responsible for the extremely high conductivity of graphene and the fuorescence of monolayer MoS2. Another effect of dimensional confnement is reduced dielectric screening between electrons and holes in semiconductors. When there is less material to screen the electric feld, there will be an increase in Coulomb interaction and more strongly bound excitons, making them more stable than excitons found in bulk materials. If the excitons are confned in a plane that is thinner than their Bohr radius (as is the case for many 2D semiconductors), quantum confnement will result in an increase in their energy compared to bulk excitons, changing the wavelength of light they absorb and emit. Their energy can be tuned somewhat by changing the number of layers in the 2D material (i.e., a bilayer structure will absorb/emit lower energy light than a monolayer). However, this can also affect the band structure, resulting in changes to other properties as well (for example, bilayer MoS2 becomes non-emissive compared to a monolayer due to changes in electronic band structure).

9.3 9.3.1

ENERGY STORAGE APPLICATIONS OF 2D NANOMATERIALS LI ION BATTERIES

From introduction of rechargeable Li-battery concept (1970s), LIBs received huge attention as a signifcant energy storage device in the last few decades. LIBs with their rechargeable nature is a signifcant energy storage devices and possess almost zero environmental impact with long lifetime and elevated energy density than other conventional rechargeable batteries. However, the utmost attainable energy density of LIBs is 250Wh kg−1, limiting their extensive utilization in recently promising uses where high-energy battery systems are needed (batterypowered electric vehicles (EVs)).57 LIBs have been an important part in transportable electronic devices because of its elevated Li-storage ability, extraordinary cyclability, and elevated reliability. Therefore, LIBs, using chemistry of LiCoO2 graphite with their cathode anode, having an estimated simulated energy density of ~380Wh kg−1, and experimentally calculated in between 120 and 150Wh kg−1, are the most important nominee for hybrid and all electrical automobile for convenience use. These obtained rates are better than other batteries, for instance, Pb-acid (~30Wh kg−1) and Ni-hydride (45–68 Wh kg−1).58 The LIB energy density

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activity rely on the chemical and physical nature of cathode and anode (e.g., conventional LIBs use graphite as anode). The low specifc capacitance (Csp) (i.e., total Ah obtainable, when a battery is discharged at a distinct discharge current, per unit weight) of graphite (372 mAh g−1)59 formulated it’s signifcance to discover alternate anodes.60 Moreover, graphite Possess low Csp (372 mAh g−1) and poor reaction rate while used in heavy-duty vehicles. On the other hand, with rising power and energy requirements for state-of-the-art electronic devices, accessible LIBs are not suffcient because of their low energy density, sluggish charging rate, short life cycle, and safety scrutinization. Till now, large research work has been done to discover new ideal anode materials to substitute graphite. So many kinds of storage are introduced, including signifcant storage for instance hydroelectric power and condensed air, plus fy wheels and electrochemical energy storage (LIBs, redox-FBs, and SCs).61

9.3.2

ZINC BASED BATTERIES

9.3.2(a) Zinc Ion Batteries The zinc ion battery (ZIB) is considered as one of the most important alternative battery chemistries to date. However, one of the challenges in ZIB development is the limitation of materials that can exhibit satisfactory Zn2+ storage. Transition metal dichalcogenides (TMDs) are widely investigated in energy-related applications due to their distinct physical and chemical properties. In particular, the wide interlayer spacings for these TMDs are particularly attractive as viable Zn2+ storage sites.62 Using MoS2 as a model system demonstrated experimentally and theoretically that even hosts with an originally poor Zn2+ diffusivity can allow fast Zn2+ diffusion. Through simple interlayer spacing and hydrophilicity engineering, the Zn2+ diffusivity is boosted by three orders of magnitude, effectively enabling the otherwise barely active MoS2 to achieve a high capacity of 232 mAh g−1, which is ten times that of its pristine form.63 The two-dimensional structural features of covalent organic frameworks (COFs) promotes the electrochemical storage of cations like H+, Li+, and Na+ through both faradaic and non-faradaic processes. However, the electrochemical storage of cations like Zn2+ ion is still unexplored although it bears a promising divalent charge. Herein, for the frst time, we have utilized hydroquinone linked β-ketoenamine COF acting as a Zn2+ anchor in an aqueous rechargeable zinc ion battery. The charge-storage mechanism comprises of an effcient reversible interlayer interaction of Zn2+ ions with the functional moieties in the adjacent layers of COF (−182.0 kcal mol−1). Notably, due to the well-defned nanopores and structural organization, a constructed full cell displays a discharge capacity as high as 276 mA h g−1 at a current rate of 125 mA g−1.64 A zincophile interphase based on 3D-printed g-C3N4 modulating interface to concurrently achieve homogeneous zinc nucleation and a dendrite-free growth. The Zn/C3N4 anode affords lower the energy barrier and more homogeneously charged distribution to facilitate highly reversible Zn plating/stripping. The symmetric Zn/ C3N4 cell presents appreciably low voltage hysteresis and superior cycling stability compared to the bare Zn. Furthermore, the Zn/C3N4/AC supercapacitor and Zn/C3N4/ MnO2 battery show long cycle stability.65

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The traditional method to fabricate a MXene-based energy storage device starts from etching MAX phase particles with dangerous acid/alkali etchants to MXenes, followed by device assembly. This is a multistep protocol and is not environmentally friendly. Herein, an all-in-one protocol is proposed to integrate synthesis and battery fabrication of MXene. By choosing a special F-rich electrolyte, MAX V2AlC is directly exfoliated inside a battery, and the obtained V2CTX MXene is in situ used to achieve an excellent battery performance. This is a one-step process with all reactions inside the cell, avoiding any contamination to external environments. Through the lifetime, the device experiences three stages of exfoliation, electrode oxidation, and redox of V2O5. While the electrode is changing, the device can always be used as a battery, and the performance is continuously enhanced. The resulting aqueous zinc ion battery achieves outstanding cycling stability (4,000 cycles) and rate performance (97.5 mAh g−1 at 64 A g−1), distinct from all reported aqueous MXene-based counterparts with pseudocapacitive properties, and outperforming most vanadiumbased zinc ion batteries with high capacity. This work sheds light on the green synthesis of MXenes, provides an all-in-one protocol for MXene devices, and extends MXenes’ application in the aqueous energy storage feld.66 9.3.2(b) Zinc Air Batteries Metal-air battery, represented by zinc air battery (ZAB), is recognized as a promising secondary battery technology with great energy density. Since the catalyst in the air-electrode must simultaneously catalyze oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), mass transfer structure of the air-electrode catalyst would signifcantly affect the ZAB performance. A facile selectively etching strategy is therefore proposed to in situ adjusting the mass transfer structure of the CoFe OER/ORR bifunctional catalyst encapsulated in graphene cage. This graphene-encapsulated CoFe catalyst with optimized mass transfer channel exhibits superior OER and ORR activity to IrO2 and Pt/C, respectively. The corresponding ZAB performance is therefore tripled compared with that equipped with Pt/C-IrO2 catalysts in terms of maximum power density (206 mW/cm2). Together with the satisfed stability, this facilely synthesized CoFe catalyst shows promising application potential in metal-air batteries. Electronically coupled metallic hybrids of NiFe layered double hydroxide nanosheet/Ti3C2 MXene quantum dots deposited on a nitrogen-doped graphene surface (LDH/MQD/NG) for high-performance fexible zinc air batteries (ZABs). As verifed from the Mott-Schottky and Nyquist plots, as well as spectroscopic, electrochemical, and computational analyses, the electronic and chemical coupling of LDH/MQD/NG modulates the local electronic and surface structure of the active LDH to provide metallic conductivity and abundant active sites, leading to signifcantly improved bifunctional activity and electrocatalytic kinetics. The rechargeable ZABs with LDH/MQD/NG hybrids are superior to the previous LDH-based ZABs, demonstrating a high-power density (113.8 mW cm−2) and excellent cycle stability (150 h at 5 mA cm−2). Moreover, the corresponding quasi solid-state ZABs are completely fexible and practical, affording a high power density of 57.6 mW cm−2 even in the bent state, and in real-life operation of tandem cells for powering various electronic devices.67

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The construction of an effcient oxygen reduction reaction and oxygen evolution reaction (ORR/OER) bifunctional electrocatalyst is of great signifcance but still remains a giant challenge for high-performance metal-air batteries. In this study, uniform FeS/Fe3C nanoparticles embedded in a porous N,S-dual doped carbon honeycomb-like composite (abbr. FeS/Fe3C@NS-C-900) have been conveniently fabricated by pyrolysis of a single-crystal Fe-MOF, which has a low potential gap ΔE of ca. 0.72 V, a competitive power density of 90.9 mW/cm2, a specifc capacity as high as 750 mAh/g, and excellent cycling stabilities over 865 h (1,730 cycles) at 2 mA/cm2 when applied as a cathode material for rechargeable zinc air batteries. In addition, the two series-linked zinc air batteries successfully powered a 2.4 V LED light as a real power source. The effcient ORR/OER bifunctional electrocatalytic activity and long-term durability of the obtained composite might be attributed to the characteristic honeycomb-like porous structure with suffcient accessible active sites, the synergistic effect of FeS and Fe3C, and the N,S co-doped porous carbon, which provides a promising application potential for portable electronic zinc air battery related devices.68 MnO2 nanorods on hexagonal boron nitride (h-BN) and their composite with high surface area carbon by a chemical method. The optimized nanocomposite catalyst (MnBN/C-75) exhibits a substantial higher onset potential (Eonset = 0.9 V vs. RHE) and limiting kinetic current density (JL = 5.6 mA cm−2) during the oxygen reduction reaction (ORR) compared to other reported h-BN-based metal-supported or metalfree electrocatalysts. Moreover, this catalyst shows a ∼4-electron transfer pathway with a low peroxide (HO2−) intermediate yield during electroreduction of oxygen, indicating a single step, frst-order kinetics as a commercial Pt/C catalyst. Besides, the mass activity of 222 mA mg−1 calculated at 0.6 V for the MnBN/C-75 catalyst is ∼21 times higher than that of MnBN (10.4 mA mg−1) and slightly lower than Pt/C (239 mA mg−1 at 0.9 V). Importantly, the MnBN/C-75 nanocomposite reveals a smaller deviation in half-wave potential (ΔE1/2 = 18 mV) compared to the Pt/C catalyst (ΔE1/2  =  50 mV) even after 5k potential cycling under similar conditions. The relatively lower ionic diffusion and charge transfer resistance at the electrode/ electrolyte interface by the MnBN/C-75 electrode support to our claim regarding a higher electrocatalytic activity. Thus, the presence of Mn3+ ions in the form of MnOOH (during composite formation) along with both h-BN support and KB carbon at the electrode surface contributes immensely in boosting the electrocatalytic activity. Thus, it could be a promising electrocatalyst if employed in the cathode compartment of low-temperature fuel cells to lead faster ORR kinetics.69

9.3.3

SODIUM ION BATTERIES

It is a rechargeable battery analogous to Li ion battery but using sodium instead of Li.70 Usually, insertion of Na+ or K+ ions in extended Ti3C2 layers are studied, which increases the difference between vertical planes.71,72 Several studies are focused on 2D nanostructure anode materials for application in SIBs. Depending on the distinct Na storage method, four different types of nanomaterials are presented, including carbonaceous materials, sodium alloys/compounds, phosphides/oxides/sulfdes/selenides, and titanium-based materials. Based on the theoretical work for interlayer

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expansion without and with functionalized group like O and F decorated- Ti3C2 MXenes with 413.0, 367.7 mAh g−1 and 151.2 mAh g−1 Na storage capacity, respectively,73 with approximately small volume changes (−0.5% to +1.6%) in sodiation and desodiation. On the other hand, the experimental results based on theoretical estimation show that the Na half cells have small reversible capacity for MXene materials, and Ti3C2 delivered a 100 mAh g−1 Csp @ i = 20 mA g−1 over 100 cycles.74 Afterward, MoS2-inserted Ti3C2Tx layers were introduced, with enhanced 250.9 mAh g−1 high Csp of over 100 cycles, and rate performance with 162.7 mAh g−1 capacitance at 1 Ag−1.75 In an extra half Na-ion cell,76 free standing Ti3C2 MXene/CNTs porous flms with volumetric capacity of 345 mAh cm−3 @ 100 mA g−1 over 500 cycles (175 mAh g−1 charging capacitance @ 20 mA g−1 after 100 cycles) are achieved. Bak et al.77 demonstrated 2D V2CTx MXene as anode material, with 78 mAh g−1 reproducible capacity up to 100 cycles at 20 mA g−1 a current density. The 3D MXenes, Ti3CTx, V2CTx, Mo2CTx flms also demonstrate improved catalytic effciency with reversible capacitance of 295, 310, 290 mAh g−1 at 2.5°C (1C = 200 mA g−1) over 1,000 cycles, enhanced compared to multilayer MXenes or their hybrids.78 Similarly, K+ intercalated Ti3C2 MXene nanoribbons also show outstanding Na/K storage, e.g., reversible capacities of 168, 136 mAh g−1 @ 20 mA g−1 and 84 mAh g−1, 78 mA hg−1 @ 200 mA g−1 in SIBs and PIBs, correspondingly.79 Moreover, Sb2O3/ MXene (Ti3C2Tx) hybrid materials provide 295 mAh g−1 rate performance at 2 mA g−1 and 472 mAh g−1 increased cycling performance subsequent to 100 cycles @ 100 mA g−1.80 Presently, the most signifcant issue is slow progress of research for electrode materials, attuned separator, and electrolyte with outstanding Na-ion diffusion, etc. For full battery system, some productive and informative efforts are performed to ensure the possibility of its realistic application. Since the huge interlayer TMDC spacing, i.e., MoSe2 (as compared to MoS2), it is suitable for accommodating cations like Na.81 Wang anode and cathode material Ti3C2/CNTs and Na0.44 MnO2, respectively, powering for ~25 min a 2.5 V LED 0.041 mW h−1 electrical energy.76 The frst charging/discharging capacitance in full cell was 270 and 286 mAh cm−3 of Ti3C2Tx/CNT-SA electrode volume, correspondingly, and preserves volumetric 242 mAh cm3 discharge capacitance more than 60 cycles @ i = 50 mA g−1, with elevated columbic effciency (99%). In one more complete SIB, 50 mAh g−1 capacitance with highest cell voltage (3.5 V) is also achieved via hard carbon as negative electrode.82 Briefy, poor anode rate capability with low capacitance is still an obstacle for SIBs. Further simulation evaluation and experimental development are needed to build up more technological data, to more regularly deal with these situations in the near future. Since the huge interlayer TMDC spacing, i.e., MoSe2 (as compared to MoS2), it is suitable for accommodating cations like Na.81 Wang et al. formed 2D MoSe2 as large as 200 nm for SIBs,83 with solid state type diffusion process; the electrochemical performance was simply showing that Na-ions can reversibly diffuse all over the nanoparticles. They also theoretically stimulated the activation barriers for the surface and interlayer reactions to be 0.34 and 1.31 eV, correspondingly, and also reported that the battery performance is because of the high SSA of the MoSe2 sample (62.3m2 g−1). Zhang et al. decorated 2D MoSe2 on MWCNTs.84 The presence of MWCNTs increased the specifc capacity up to 20% and enhanced the cyclability for longtime performance. Shi et al. customized the MoSe2 composition via doping S synthesizing Mo(Se0.85S0.15)2,

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but the battery performance was analogous to MoSe2.85 Kang et al. formed yolkshell-shaped MoSe2 microspheres. In the vertically formed MoSe2 nanosheets, the counter-ions could directly diffuse along the 2D structure of MoSe2. Therefore, the battery performance was outstanding in contrast with other reported SIBs and LIBs, showing familiar fat plateau during both charging and discharging. The similar group also reported fullerene-like MoSe2, presenting an outstanding structural stability in the SIBs’ performance.86 As the Na incorporation resultant change in structure to form metallic Mo and Na2Se (similar case for LIBs), carbon nanotubes built the fullerene-like cage via keeping the electroactive materials intact. So the electrode kept its capacitance till 50 cycles.87 Yajun Zhao et al., for the frst time, studied CoFe LDH with nitrates in its interlayer as an intercalation-type anode material for SIBs. As compared to well-established conversion reaction for MHOs in LIBs, an extraordinary intercalation/de-intercalation mechanism for Na+ storage was exposed. The CoFe-NO3-LDH anode has frst 498 mAh g−1 discharge capacity and 209 mAh g−1 reversible capacity after 200 cycles at 1 A g−1. The excellent Na+ storage effciency is credited to the pillared interlayer nitrates expanded in between the CoFe-LDH interlayer distance, helping the intercalation/de-intercalation of Na+-ions. Moreover, the LDH material topochemical transformation property assured high structural stability even with the change in valence of Co2+/Co2−x and Fe2+/Fe3+ in their host layers. Additionally, the CoFe-NO3-LDH interlayer space obviously shows that the LDH is a good Na+-ion conductor.88 Bare oxide anodes usually suffer from bad rate capability and poor cyclability due to their very large changes in volume in the Na insertionextraction process. To overcome such issues, one method is to use 2D nanosheets because it can give short paths for ion transport, e.g., ultrathin NiO nanosheets (thickness about 4–5 nm) synthesized by facile solvothermal method followed by annealing in air. For Na storage, the NiO nanosheets deliver a high reversible specifc capacity of 299 mA h g−1 at a current density of 1 A g−1, and the capacity still remains as high as 266 mA h g−1 after the 100th cycle. Recently, Yu et al. studied a two-step method for 2D TMO nanosheet synthesis with variable pore sizes via GOs as a sacrifcial template.89,90 This approach has been demonstrated for the synthesis of various 2D TMO nanosheets, counting simple oxides for instance Co3O4, Fe2O3, Mn2O3, and mixed oxides, for example, ZnMn2O4 (ZMO), ZnCo2O4 (ZCO), NiCo2O4 (NCO), and CoFe2O4 (CFO). Particularly, 2D TMO nanosheets show greatly enhanced rate capability and cycle stability for both Li- and Na-ion storage because of the increased SSA and interfaces, facile interfacial transport, and small diffusion paths.91 One more plan is to combine the oxide with a 2D support, for example, graphene, carbon cloth, and stainless steel mesh that can improve the conductivity. A variety of oxides (e.g., Fe2O3, Fe3O4, Co3O4, NiO, CuO, MoO3, Mn3O4, SnO2, and SnO) have been studied as SIB anodes. Based on those studies, the Sn-based oxides, for example, SnO2 and SnO, attracted particular awareness due to their high hypothetical reversible capacity, reasonable operating voltage, excellent performance, and low-cost. For instance, Wang et al. studied SnO2 nanosize particles (5 nm) anchor on 2D-rGO and the synthesized electrode with capacitance of 302 mAh g−1 over 100 cycles at 160 mA g−192. In recent times, Pei et al. showed that 3D SnO2 @G (SnO2@3DG) has improved Na storage than its 2D equivalent (SnO2@2DG), which is due to its high

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SSA and 3D porous structural design.93 The SnO2@3DG based anode illustrated a superior reversible capacitance of 432 mAh g−1 after 200 cycles at 100 mA g−1. Moreover, the controlled SnO atomic layer nanosheets were synthesized on carbon cloth. The thinnest SnO nanosheet anodes (2–6 SnO monolayers) showed good performance. Particularly, an initial charge and discharge capacity of 848 and 1072 mAh g−1 was calculated, correspondingly, at 0.1 A g−1. Additionally, inspiring reversible capacity of 665 mAh g−1 after 100 cycles at 0.1 A g−1 and 452 mAh g−1 after 1,000 cycles at 1.0 A g−1 was calculated, with outstanding rate performance.91

9.3.4

SUPERCAPACITORS

Supercapacitors are one of the most promising energy storage devices, which have been attracting a lot of interest because of their advantages, including high power density, high capacitive retention, fast charge/discharge rate, and long cyclic life. The main constituents of supercapacitors are electrode and electrolyte materials, current collectors, separators, and sealants, which play an important role in the electrochemical performance of the supercapacitor devices. Among all, the selection of electrode materials and their design play an essential role in improving the capacitance performance of supercapacitors, in which the electrodes should be able to deliver a high specifc surface area, corrosion resistance, conductivity, as well as a good thermal and chemical stability. Furthermore, they should be low-cost and environmentally friendly. Also, their ability to transfer the faradic charge is essential to increase the total capacitance value.94 Another effective factor is the size of the pores, for instance, Y. Gogotsi et al.95 have studied the infuence of the pore size on the specifc capacitance of carbide-derived carbon material, and they reported that increasing the pore size lesser than 2 nm can provide more capacitance and, consequently, higher energy density, whereas growing the pore size greater than 2 nm led to decreasing the specifc capacitance. However, the smaller the pore, the greater the equivalent series resistance (ESR) and hence decreasing the power density. According to this, the selection of the electrode materials also depends upon their application. For instance, for the applications in which having more peak currents is essential, the electrode materials should have larger pores, while materials with smaller pores make the electrodes suitable for the purposes that require developed energy density.94 Supercapacitors are categorized into the electrical double-layer capacitors (EDLCs) and the pseudocapacitors based on their energy storage mechanism. The charge storage mechanism in EDLCs is according to the electrosorption of ions and formation of an electrochemical double layer (EDL); however, the pseudocapacitors store the charges through the faradic reactions (redox reactions). The electrode materials for supercapacitors are classifed based on three main categories, including carbonbased materials (e.g., carbon nanotubes (CNTs), activated carbon materials (ACs), and graphene), transition metal oxides (e.g., ruthenium oxide (RuO2), manganese oxide (MnO2), and nickel oxide (NiO)), and conducting polymers (e.g., polyaniline (PANI), polythiophene (PTh), and polypyrrole (PPy)). The charge storage mechanism for nanostructured carbon-based supercapacitor is based on the electrochemical double layer that formed between the electrode and the electrolyte interface, while

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conducting polymers and transition metal oxides are known as pseudocapacitive materials of their ability to provide the fast-reversible redox reactions.96,97 Novel twodimensional (2D) materials have shown many potentials to be used in different felds, such as environmental sensing,98 water treatment,99 and energy conversion/storage100 because of their extraordinary optical, physical, chemical, and electronic properties. There are different 2D materials/2D based composites for electrodes in supercapacitors, such as MXenes, transition metal dichalcogenides (TMDs), and black phosphorus (BP). MXenes MXenes are a promising candidate in the feld of energy storage, especially as electrodes for supercapacitors applications, because of their unique combination of hydrophilicity and metallic conductivity.101 There are several examples of MXene, such as Ti2CTx, Ti3C2Tx, V2CTx, Nb4C3Tx, Nb2CTx, Ta4C3Tx, and Ti2NTx MXene, which can be used as electrode materials for supercapacitors due to their capability of storing the charges through pseudocapacitive mechanisms.102,103 Qiu et al. have synthesized Ti3C2 MXene through different methods and reported a conductivity of 260 S cm−1, which is two orders of magnitude greater than that of rGO with conductivity about 1 S cm−1.104 The electrical conductivity of Ti3C2 MXene flms attained through the spin-cast method was reported up to 6500 S cm−1, which is higher than most of the 2D materials.100 The intercalation of multivalent, such as Mg2+, Al3+, or Zn2+, and the large organic ions is one of the important parameters, whereas controlling the ion dynamics between MXene layers leads to achieving a high power in supercapacitors, which provides MXene-based supercapacitors with a low resistance to replace the electrolytic capacitors.100 Zhang et al.105 have prepared a conductive and highly deformable freestanding electrode by designing the composite of MXene and MnO2 on carbon cloth. The resultant electrode showed an enhanced specifc capacitance of about 511.2 F g−1 in comparison with MnO2 nanorods/carbon cloth, which exhibited 302.3 F g−1. Also, it was reported that the enhanced capacitance and high rate ability are because of the synergistic coupling of MnO2 and MXene. Furthermore, the fabricated asymmetric fexible device made by MnO2 nanorods, MXene, and carbon cloth has shown large energy and power density of 29.58 Wh kg−1 and 749.92 W kg−1, respectively. Besides the advantages of having an ultrahigh conductivity and a hydrophilic surface, MXenes material also exhibited high mechanical and chemical stability.101 The structural stability and conductivity of active electrode materials can be enhanced by means of effcient methods provided by interfacial engineering. For example,106 Ti3C2 MXene was introduced to the fexible Ni2Co-LDHs nanoarrays to enhance the mechanical stability of the structure. The battery-type supercapacitors fabricated by the resultant composite exhibited ultrahigh long-life stability as well as a good rate capability (126 mAh g−1 at 150 A g−1), which was approximately 5.7 times higher than that of pure Ni2Co- LDHs (22 mAh g−1). The enhanced intrinsic performance is due to the excellent conductivity and the strong interfacial interactions of AL-Ti3C2 MXene composite (layered Ti3C2 MXene, which was obtained by etching the Al layers from the primary Ti3AlC2 MAX). Furthermore, the Ti3C2 MXene showed a high Young’s modulus of 330 GPa that is greater than those of other prepared Nico-LDH nanosheets such as FeCo@NiCo-LDH reported by L. Gao.107 The vanadium carbide (V4C3) MXenes were prepared through etching the intermediate

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aluminum layer from their precursors (vanadium aluminum carbide MAX phase) using HF. Etching the Al provides the path, which eases the mobility of the ions from the electrolyte into the electrode, thus increasing the electrochemically active sites. The resultant electrode showed a high specifc capacitance (330 F g−1 at 5 mV s−1) as well as high capacitive retention of 90% even after 3,000 cycles.108 Furthermore, the pure Ti3C2Tx and Ti3C2Tx/MWCNT composite flms were tested along with rGO electrodes in asymmetric supercapacitors consisting of different electrolytes. The asymmetric supercapacitors with aqueous electrolyte provided the specifc capacitances (48 F g−1 and 78 F cm−3), which were lower than those values achieved by asymmetric cell based on Ti3C2Tx electrodes. However, the reported potential window and, consequently, the energy density for the asymmetric device were higher than that of the symmetric cell. The gravimetric and volumetric cell capacitances of organic asymmetric supercapacitor reached to 30 F g−1 and 41 F cm−3, respectively, which were the symmetric cells fabricated by Ti3C2Tx and rGO. The potential window and, consequently, the energy density of the asymmetric supercapacitor with organic electrolytes delivered higher values in comparison with the asymmetric aqueous device.109 Also, Rakhi et al.110 have synthesized the nanocrystalline-MnO2 directly on MXene nanosheets using a facile chemical method. The aqueous pseudocapacitor fabricated using MnO2/Ti3− C2Tx−Ar composite demonstrated a higher specifc capacitance (212 F g−1) compared to pure Ti3C2Tx−Ar. Furthermore, the symmetric supercapacitors, fabricated using the resultant composite as electrode materials, showed higher capacity retention (approximately 88% after 10,000 cycles) compared to the symmetric capacitor containing the pure −MnO2-based electrodes. Transition Metal Dichalcogenides (TMDs) Transition metal dichalcogenides (TMDs) are layered compounds with the crystal structure formulae of MX2, in which M is a transition metal element, for instance, Ti, Mo, V, W, Re, Ta; and the term X can be any chalcogenide element, such as S, Se, Te. The TMD monolayers stack together by the van der Waals force, and each layer comprises three atomic layers, in which a transition metal layer is inserted between two chalcogen sheets.111 The TMDs such as TiS2, MoS2, TaS2, and NbSe2 are the layered structure similar to the graphite and the frst graphene descendants that are almost comparable to graphene in terms of their fexible, thin, and transparent structure.112 However, in contrast with graphene, many 2D TMDs are semiconductor in nature, and they exhibit a high potential to be made into low power and ultrasmall transistors that are more effcient and successful than silicon-based transistors to deal with ever-shrinking devices.113 TMDs have attracted a lot of interest as electrode materials due to their excellent physicochemical properties, such as large surface area, high catalytic activity, good semiconducting ability, low-cost, high chemical stability, high mechanical properties, their easy synthesis through various preparation methods,114,115 such as mechanical exfoliation, direct sonication in solvents, chemical vapor deposition (CVD), etc. 2D TMDs are versatile materials applied in various application felds, including sensing photonics, electronics, and energy devices114,115 such as sensors, optical displays, transistors, and supercapacitors due to their thin atomic profle, which leads to having ideal conditions to achieve higher electrostatic effciency, a tunable electronic structure, mechanical strength, optical

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transparency, and sensor sensitivity.116 For instance, MoS2 nanosheets as direct bandgap materials are attractive semiconductors, in which two S atoms and one molybdenum (Mo) occupy two various sublattices of the hexagonal MoS2 structure, and the Mo layer is inserted between two sulfur layers.117 MoS2 can be used in optoelectronic devices and fexible electronic as well as for composite flms due to their superior elastic properties of single-layer and multilayer.115 In the case of energy storage development, the 2D layered applications have been enormously increased due to their superior structure with interspace between layers and high surface area, which provides easier intercalation for ions and small molecules and leads to the reversible and quick redox reactions.118 For example, the asymmetric supercapacitor was fabricated using AC as the negative electrode and Co@NiSe2 material with a nanowire structure as the positive electrode that was obtained through a simple hydrothermal method. The resultant electrode showed a high specifc capacitance of 3167.6 Fg−1 at a current density of 1 A g−1, as well as a high energy density of 50 Wh kg−1 at a power density of 779 W kg−1. Also, 79.4% of initial capacitance remained after 4,000 cycles.119 Besides all the advantages, the interactions between the 2D TMDs layers and their high surface energy lead to an increase in the restacking possibility of the layers, thus reducing the number of active sites.120 One of the effcient solutions to address the mentioned disadvantages and increase the TMD activity is to synthesize the new functional hybrid materials, which consist of 2D TMDs and a highly conductive supporting material.121,122 For instance, the hybridization of 2D TMDs with different carbon-based supporting materials to increase the active sites and ease the charge transferability has been reported in several numbers of research.121,123,124 Over the last several years, the heterostructure hybrid of 2D TMDs with graphene sheets has offered signifcant potential for various applications due to their electronic and enhanced physicochemical properties. There are different approaches to synthesize the 2D TMDs/Gr hybrids with specifc characteristics, such as high specifc surface area, unique morphology, thickness, and electroactive site numbers, according to their application purposes, including (a) chemical vapor deposition synthesis,125 (b) thermal and chemical reduction synthesis,126 (c) electrochemical synthesis,127 (d) microwave-assisted synthesis,128 (e) hydrothermal and solvothermal synthesis,129 etc. Furthermore, for the frst time, the electrochemical supercapacitor performance of a VSe2/RGO hybrid was reported, in which the hybrid supercapacitor was synthesized through the one-step hydrothermal method. The supercapacitor showed a specifc capacitance of about 680 F g−1 at a normalized mass current of 1 A g−1, as well as a high energy density of about 212 Wh kg−1 at a power density of about 3.3 kW kg−1. Also, 81% of retention was reported almost even after 10,000 cycles.130 Black Phosphorus (BP) Black phosphorus is a p-type semiconducting layered structure with narrow and direct bandgap.131 BP has shown great potential owing to its high theoretical specifc capacitance (2596 mAh g−1), controllable bandgap (0.302.2 eV) by regulating the number of layers of the BP (phosphorene),131 as well as the high carrier mobility as high as 1,000 cm2/Vs132 for building optoelectronic technologies.131,133 Also, the high volumetric capacitance of 13.75F cm−3 has been obtained, exploiting liquid-exfoliated BP nanofakes as electrode materials for fexible all-solid-state supercapacitor

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(ASSP), which is higher than that of restacked graphene (1.0 F cm−3)134. Also, BP materials are a suitable candidate as electrode material for EDLCs. The typical electrochemical double-layer capacitance of BP material is due to the large space between BP layers, which leads to the rapid ion diffusion within the BP layers.135,136 Hao et al.137 have assembled a fexible asymmetric supercapacitor using the liquidexfoliated BP nanofakes that reached to a large charging-discharging rate of 10 V s−1 and provided a volumetric capacitance of 13.75 F cm−3, Ed of 2.47 mWh cm−3, Pd of 8.83 W cm−3, and good cycling stability of over 30,000 cycles. Apart from the energy storage applications, the chemical activities and high surface area make them a suitable candidate to be used as absorbents, catalytic agent.137 Yang et al.136 have fabricated composite flm using black phosphorus and CNT (ratio of 1:4) as electrodes for supercapacitor. The fabricated supercapacitor showed a volumetric specifc capacitance of 41.1 F cm−3 at 0.005 V s−1, a high energy density of 5.71 mWh cm−3, and a high power density of 821.62 W cm−3 as well as superior mechanical fexibility. In addition, 91.5% of initial capacitance remained after 10,000 cycles.

9.3.5

HYBRID ENERGY STORAGE (HES) DEVICES

Perfect ESDs are those that have elevated energy and power. Although major progress has been made in raising energy amount of SCs, still their energy is low. The HES system intends to achieve similar effciency to batteries regarding energy storage and in keeping the elevated power nature with good cycle life, like SCs. The HES devices can get through hybridization at material level (HDL/HML) or device, called internal and external hybrids, correspondingly. For device-based, EDLC and a battery might be included in tandem in modules and need power conversion methodology to separately handle both EES systems. Based on these necessities, HDL-based HES systems are multicomponent and usually experience fabrication diffculty, are not cost-effcient, and have high weight/volume. Recently, progress has been made for energy storage to achieve battery-level energy, SC-level power, and cyclability in a single device. The HML system becomes more advanced in improving the device performance. The HML system includes one battery-like charge accumulation and one capacitive (EDLC or PCs) charge storage procedure (Figure 9.6). For instance, in earlier reports, hybridization of an asymmetric SC based on activated carbon (SC electrode) with zinc (battery electrode), in a parallel combination. In this process, the activated carbon acts as a negative electrode of SCs, and Zn as negative electrode of secondary battery, and self-doped polyaniline acts as a collective positive electrode. As the working potential window of both material coincides, the HES devices can get high-energy nature like battery and the high-power ability like SCs. Hybrid ion capacitors in energy are intermediary in between batteries and SCs but, by theory, showing SC-like power ability and cyclability.61

9.3.6

FUEL CELLS

A number of 2D materials (e.g., graphene,139,140 borophene,141,142 and MXene143,144) have been recognized as materials with high surface area and great electric conductivity, which makes them suitable for their application in catalysis. MAB, a group

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FIGURE 9.6

Emerging 2D Materials and Applications

Illustration of HES systems with capacitive and battery-type electrode.

of 2D ceramic/metallic boride (instead of C or N in MXenes) materials with similar structure to MXenes, also considered as excellent candidates for electrocatalysis owing to the exceptionally high conductivity, rich interlayer porosity, high surface area, adjustable bandgap structure, etc. These materials were found to possess high heterogeneous electron transfer (HET) rates and promising electrocatalytic performances toward hydrogen evolution reaction (HER) and oxygen reduction reaction (ORR)145 and other electrochemical energy storage systems.146 Two-dimensional (2D) materials such as graphene and hexagonal boron nitride (h-BN) can be used as robust and fexible encapsulation overlayers, which effectively protect metal cores but allow reactions to occur between inner cores and outer shells. Electrochemical (EC) tests combined with operando EC-Raman characterizations were performed to monitor the reaction process and its intermediates, which confrm that Pt-catalyzed electrocatalytic processes happen under few-layer h-BN covers. The confnement effect of the h-BN shells prevents Pt nanoparticles from aggregating and helps to alleviate the CO poisoning problem. Accordingly, embedding nanocatalysts within ultrathin 2D material shells can be regarded as an effective route to design high-performance electrocatalysts.147 The design of new cathode materials is vital to develop the high performance proton exchange membrane fuel cell (PEMFC). The adsorption properties of the species involved in the oxygen reduction reaction (ORR) and several ORR mechanisms on phosphorene were presented using the frst-principle calculations. Results show

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that the H adsorption is more preferable than O2, and the OOH spontaneously form from the adsorbed H and O2. The HOOH from the hydrogenation of the OOH species could be further hydrogenated into OH and H2O, and the OH would be fnally hydrogenated into the (second) H2O. The reaction barrier of rate-determining step is 1.02 eV. Moreover, the presented kinetic processes are confrmed by the thermodynamic simulation from Gibbs free energy calculations and molecular dynamics simulations. These results indicate that phosphorene as one of new 2D materials could be a promising metal-free cathode material for PEMFC.148 Development of bifunctional catalysts with low platinum (Pt) content for the ethanol oxidation reaction (EOR) and the oxygen reduction reaction (ORR) is highly desirable yet challenging. Herein, we present structural engineering of a series of twodimensional/three-dimensional (2D/3D) hierarchical N-doped graphene-supported nanosized Pt3Co alloys and Co clusters (PtCo@N-GNSs) via a hydrolysis-pyrolysis route. For the ORR, the optimal PtCo@N-GNS exhibits a high mass activity of 3.01 A mgPt−1, which is comparable to the best Pt-based catalyst obtained through sophisticated synthesis. It also possesses excellent stability with minor decay after 50,000 cyclic voltammograms (CV) cycles in acidic medium. For the EOR, PtCo@N-GNS achieves the highest mass-specifc and area-specifc activities of 1.96 A mgPt−1 and 5.75 mA cm−2, respectively, among all the reported EOR catalysts to date. The unique 2D/3D hierarchy, high Pt utilization, and valid encapsulation of nanosized Pt3Co/ Co synergistically contribute to the robust ORR and EOR activities of the present PtCo@N-GNS. A direct ethanol fuel cell based on PtCo@N-GNS delivers a high open-circuit potential of 0.9 V, a stable power density of 10.5 mW cm−2, and an excellent rate performance, implying the feasibility of the bifunctional [email protected] A conventional carbon felt (CF) electrode was modifed by NiFe2O4 (NiFe2O4@CF), MXene (MXene@CF), and NiFe2O4-MXene (NiFe2O4-MXene@CF) using facile dip-and-dry and hydrothermal methods. In these modifed CF electrodes, the electrochemical performance considerably improved, while the highest power density (1,385 mW/m2), which was 5.6, 2.8, and 1.4 times higher than those of CF, NiFe2O4@ CF, and MXene@CF anodes, respectively, was achieved using NiFe2O4-MXene@ CF. Furthermore, electrochemical impedance spectroscopy and cyclic voltammetry results confrmed the superior bio-electrochemical activity of a NiFe2O4-MXene@ CF anode in a MFC. The improved performance could be attributed to the low charge transfer resistance, high conductivity, and number of catalytically active sites of the NiFe2O4-MXene@CF anode. Microbial community analysis demonstrated the relative abundance of electroactive bacteria on a NiFe2O4-MXene@CF anodic bioflm rather than CF, MXene@CF, and NiFe2O4@CF anodes.150 MoS2 is a promising H2 evolution catalyst with excellent kinetics for driving the hydrogen evolution reaction (HER). The catalytic effects were suggested to stem from the sulfur edges of MoS2 plates while the basal planes were catalytically inert.151 Therefore the surface area and the crystallinity of the catalysts play an important role in the H2 activation property. Promising results have been reported using MoS2 nanoparticles with a high concentration of edges. As shown in Figure 9.4 (a), Dai’s group has demonstrated a high HER effciency using MoS2graphene oxide composites as the catalysts. MoS2-graphene hybrid nanomaterials can be obtained by a one-step solvothermal reaction of (NH4)MoS4 and hydrazine

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in an N,N-dimethylformamide (DMF) solution of mildly oxidized graphene oxide at 200°C. The MoS2 synthesized on graphene surface has abundant open edges, which facilitate the electrochemical reaction for H2 generation.127

9.4 CONCLUSION In this chapter, we try to explain the development of two-dimensional materials in terms of its morphology, surface area, and electrical conductivity, where we picked commonly employed materials like graphene, hexagonal boron nitride, transition metal chalcogenides, phosphorene, and MXene. We tried to explain their property dependence on the morphology and its importance in energy storage devices like Na/Li batteries, supercapacitors, and fuel cells. Along with this, we also focused on their synthesis mechanisms developed in literature and property dependence on these mechanisms.

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119. Jiang, J. W. & Park, H. S. Mechanical properties of MoS2/graphene heterostructures. Appl Phys Lett (2014). doi:10.1063/1.4891342 120. Muhammad Yousaf et al. Advancement in layered transition metal dichalcogenide composites for lithium and sodium ion batteries. J Electr Eng (2016). doi:10.17265/2328–2223/2016.02.003 121. Toth, P. S. et al. Asymmetric MoS2/graphene/metal sandwiches: Preparation, characterization, and application. Adv Mater (2016). doi:10.1002/adma.201600484 122. Zhang, X. et al. MoS2/Carbon nanotube core-Shell nanocomposites for enhanced nonlinear optical performance. Chem—A Eur J (2017). doi:10.1002/chem.201604395 123. Liu, X. et al. Rotationally commensurate growth of MoS2 on epitaxial graphene. ACS Nano (2016). doi:10.1021/acsnano.5b06398 124. Jiang, L. et al. Monolayer MoS2-graphene hybrid aerogels with controllable porosity for lithium-ion batteries with high reversible capacity. ACS Appl Mater Interfaces (2016). doi:10.1021/acsami.5b10692 125. Li, X., Zhang, L., Zang, X., Li, X. & Zhu, H. Photo-promoted platinum nanoparticles decorated MoS2@graphene woven fabric catalyst for effcient hydrogen generation. ACS Appl Mater Interfaces (2016). doi:10.1021/acsami.6b01903 126. Qin, W. et al. MoS2-reduced graphene oxide composites via microwave assisted synthesis for sodium ion battery anode with improved capacity and cycling performance. Electrochim Acta (2015). doi:10.1016/j.electacta.2014.11.034 127. Li, Y. et al. MoS2 nanoparticles grown on graphene: An advanced catalyst for the hydrogen evolution reaction. J Am Chem Soc (2011). doi:10.1021/ja201269b 128. MarriThese Authors Contributed Equally., S. R., Ratha, S., Rout, C. S. & Behera, J. N. 3D cuboidal vanadium diselenide embedded reduced graphene oxide hybrid structures with enhanced supercapacitor properties. Chem Commun (2017). doi:10.1039/ c6cc08035a 129. Xia, X. H., Chao, D. L., Zhang, Y. Q., Shen, Z. X. & Fan, H. J. Three-dimensional graphene and their integrated electrodes. Nano Today (2014). doi:10.1016/j. nantod.2014.12.001 130. Zheng, Y. et al. Recent advances of two-dimensional transition metal nitrides for energy storage and conversion applications. FlatChem (2020). doi:10.1016/j.fatc.2019.100149 131. Wang, H. et al. Ultrathin black phosphorus nanosheets for effcient singlet oxygen generation. J Am Chem Soc (2015). doi:10.1021/jacs.5b06025 132. El-Kady, M. F. & Kaner, R. B. Scalable fabrication of high-power graphene micro-supercapacitors for fexible and on-chip energy storage. Nat Commun (2013). doi:10.1038/ ncomms2446 133. Wu, S., Hui, K. S. & Hui, K. N. 2D Black Phosphorus: from Preparation to Applications for Electrochemical Energy Storage. Adv Sci (2018). doi:10.1002/advs.201700491 134. Yang, B. et al. Flexible black-phosphorus nanofake/carbon nanotube composite paper for high-performance all-solid-state supercapacitors. ACS Appl Mater Interfaces (2017). doi:10.1021/acsami.7b13572 135. Hao, C. et al. Flexible all-solid-state supercapacitors based on liquid-exfoliated blackphosphorus nanofakes. Adv Mater (2016). doi:10.1002/adma.201505730 136. Yasaei, P. et al. Stable and selective humidity sensing using stacked black phosphorus fakes. ACS Nano (2015). doi:10.1021/acsnano.5b03325 137. Giorgi, G., Fujisawa, J. I., Segawa, H. & Yamashita, K. Small photocarrier effective masses featuring ambipolar transport in methylammonium lead iodide perovskite: A density functional analysis. J Phys Chem Lett (2013). doi:10.1021/jz4023865 138. Noori, A., El-Kady, M. F., Rahmanifar, M. S., Kaner, R. B. & Mousavi, M. F. Towards establishing standard performance metrics for batteries, supercapacitors and beyond. Chem Soc Rev (2019). doi:10.1039/c8cs00581h

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139. Sun, J. et al. Regulation of electronic structure of graphene nanoribbon by tuning longrange dopant-dopant coupling at distance of tens of nanometers. J Phys Chem Lett (2020). doi:10.1021/acs.jpclett.0c01839 140. Suter, J. L., Sinclair, R. C. & Coveney, P. V. Principles governing control of aggregation and dispersion of graphene and graphene oxide in polymer melts. Adv Mater (2020). doi:10.1002/adma.202003213 141. Mannix, A. J., Zhang, Z., Guisinger, N. P., Yakobson, B. I. & Hersam, M. C. Borophene as a prototype for synthetic 2D materials development. Nat Nanotechnol (2018). doi:10.1038/s41565-018-0157-4 142. Kiraly, B. et al. Borophene synthesis on Au(111). ACS Nano (2019). doi:10.1021/ acsnano.8b09339 143. Tang, X., Guo, X., Wu, W. & Wang, G. 2D Metal carbides and nitrides (MXenes) as high-performance electrode materials for lithium-based batteries. Adv Energy Mater (2018). doi:10.1002/aenm.201801897 144. Xu, C. et al. MXene (Ti3C2Tx) and carbon nanotube hybrid-supported platinum catalysts for the high-performance oxygen reduction reaction in PEMFC. ACS Appl Mater Interfaces (2020). doi:10.1021/acsami.0c02446 145. Wang, J. et al. Discovery of hexagonal ternary phase Ti2InB2 and its evolution to layered boride TiB. Nat Commun (2019). doi:10.1038/s41467-019-10297-8 146. Rosli, N. F. et al. MAX and MAB Phases: Two-dimensional layered carbide and boride nanomaterials for electrochemical applications. ACS Appl Nano Mater (2019). doi:10.1021/acsanm.9b01526 147. Duan, Y. e. et al. Cobalt nickel boride nanocomposite as high-performance anode catalyst for direct borohydride fuel cell. Int J Hydrogen Energy (2021). doi:10.1016/j. ijhydene.2021.02.064 148. Lu, Z. et al. Phosphorene: A promising metal free cathode material for proton exchange membrane fuel cell. Appl Surf Sci (2019). doi:10.1016/j.apsusc.2019.02.013 149. Gao, J. et al. MOF-Derived 2D/3D hierarchical N-doped graphene as support for advanced Pt utilization in ethanol fuel cell. ACS Appl Mater Interfaces (2020). doi:10.1021/acsami.0c15493 150. Tahir, K. et al. Nickel ferrite/MXene-coated carbon felt anodes for enhanced microbial fuel cell performance. Chemosphere (2021). doi:10.1016/j.chemosphere.2020.128784 151. Kibsgaard, J., Chen, Z., Reinecke B. N. & Jaramillo, T. F. Engineering the surface structure of MoS2 to preferentially expose active edge sites for electrocatalysis. Nature Mater (2012)

10

Role of 2D Materials in Environmental Monitoring Renu Dhahiya, Moumita Saha, Ashok Kumar, Pankaj Sharma, Ram Sevak Singh, Varun Rai, and Kamalakanta Behera

CONTENTS 10.1 10.2 10.3

Introduction..................................................................................................195 Unique Features of Two-Dimensional Materials in Sensing .......................196 Classifcation of Two-Dimensional Materials Based Environmental Sensors .........................................................................................................197 10.3.1 Graphene.........................................................................................197 10.3.2 Transition Metal Dichalcogenides..................................................198 10.3.3 Black Phosphorus (BP)...................................................................198 10.3.4 2D Metal Oxides ............................................................................199 10.3.5 2D Metal-Organic Framework .......................................................199 10.3.6 MXenes Based 2D Materials......................................................... 200 10.4 Monitoring of Environmental Contaminants ............................................... 200 10.4.1 Heavy Metals..................................................................................201 10.4.2 Organic Compounds...................................................................... 202 10.4.3 Pesticides ....................................................................................... 203 10.4.4 Antibiotic Compounds................................................................... 204 10.4.5 Bacteria.......................................................................................... 206 10.4.6 Gases.............................................................................................. 207 10.4.7 Volatile Organic Compounds ........................................................ 208 10.5 Future Perspective and Conclusions ........................................................... 209 References...............................................................................................................210

10.1

INTRODUCTION

The improvement in the living standards of living beings and the Industrial Revolution has led to environmental degradation to its peak. Today the most important concerns are to protect our environment for the betterment of human beings and other living creatures. However, due to several anthropogenic activities, the environment is loaded with various types of direct and indirect pollution-causing substances, such as heavy metals, toxic gases, volatile organic compounds, persistent organic DOI: 10.1201/9781003247890-10

195

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pollutants, and pesticides. These pollutants have a hazardous impact on the ecosystem and ultimately on human beings.1,2 Heavy metal is the most released pollutant in water and soil, which chiefy includes copper, iron, mercury, arsenic, etc. Owing to their nonbiodegradability, heavy metal ions accumulate in the living system due to biomagnifcations. Moreover, these ions are carcinogenic and cause several other diseases, such as damage to kidneys, lungs, liver, esophagus, stomach, prostate, and skin.3 Another major source of environmental degradation is poisonous gases such as oxides of nitrogen (NOx), methane gas (CH4), hydrogen sulfde (H2S), carbon monoxide, and carbon dioxides. These poisonous gases are released from various point and nonpoint sources, such as power plants, transport vehicles, factories, agricultural activities, forest fres, volcanic eruptions, and waste treatments. These gases cause serious ailments such as lung cancer, heart disease, and both acute and chronic respiratory diseases, such as asthma. The release of these gases also results in global warming and ozone degradation.4 The next class of environmental pollutants includes pesticides. These are the chemicals that are meant to control the growth of weeds and undesirable plants that decline the crop yield. After the green revolution, the use of chemicals in agriculture has boomed to a new height. These chemicals include organophosphates, organochlorines, carbamates, and thiocarbamates. These chemical pesticides along with their benefts have various hazards also, which includes the degradation of aquatic plants and animals and several death-causing diseases in human beings. Antibiotics, which include tetracyclines, aminoglycosides, macrolides, b-lactams, vancomycin, fuoroquinolone, phenols, etc., are the overall utilized drugs that are used against pathogenic microorganisms in living organisms. Moreover, these are also the major contributors to environmental pollution. These antibiotics cause ecosystem damage and disturbance in the function of algae communities.5 Hence, we can say that the monitoring of the environment is the major concern for today’s world. Several metal particles discarded into the environment stay undecayed or exist forever as sophisticated compounds. This has grown exponentially with contemporary civilization. It has a negative infuence on both human health and the environment. As a result, developing a high-speed, sensitive, and fundamental demonstration approach for detecting and monitoring metal particles’ toxicity in soil and water is critical. Nowadays, ultrasensitive 2D nanocomposites adapted terminals are used mostly to identify metals such as mercury, cadmium, lead, copper, iron, and zinc with incredible accuracy.6,7

10.2 UNIQUE FEATURES OF TWO-DIMENSIONAL MATERIALS IN SENSING Evolving 2D materials have sparked a revolution in electronics, photonics, optical, thermal, and sensors. These are inherited with some unique properties, which makes them apt material for sensing application. These properties incorporate simplicity of functionalization, enormous surface area, high chemical stability, high electrical conductivity, and high mechanical strength due to associated structural resemblance with graphene.8–10 The 2D material possesses a higher surface-to-volume ratio and ultrathin planer surface than 0D, 1D, and 3D materials, making it suitable for enhanced performance in the sensor. Due to huge surface area and vigorous surface

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Layered materials

Graphene family

2D chalcogenides

2D oxides

FIGURE 10.1

Graphene

Fluorographene

Graphene oxide

h-Bn “white graphene”

BCN

MoS2, WS2, MoSe2, WSe2,

Semiconducting dichalcogenides: MoTe2, WTe2, ZrS2

Layered semiconductors: GaSe, GaTe, InSe, BiSe

Metalic dichalcogenides: NbSe2, NbS2, TaS2, TiS2, NiSe2

Micas BSCCO

Layered Cu oxides

MoO3, WO3,

TiO2, MnO2, V2O5, TaO2, RuO2

Perovskite-type LaNb2O7, Ca2Nb3O10

Hydroxides: Ni(OH)2, Eu(OH)2

materials are stable in ambient room temperatures

potentially stable materials in ambient cinditions

materials are stable only in an inert atmosphere

3D compounds that have been exfoliated into monolayers

The classification of layered materials based stability.12

activities, graphene and other 2D materials demonstrate extraordinary applications in gas monitoring. The material used in sensing equipment requires large tensile strength, and metal oxide nanomaterials possess some characteristics that make them suitable for use in sensing applications. Their high surface-to-volume ratio makes more surface area available for analytes binding, allowing for more rapid and sensitive detection. Apart from these collective properties, some 2D materials also have exceptional properties, such as superior thermal transport of BN nanosheets, tuned energy bandgap, and high reactivity of molybdenum disulfide (MoS2) toward hydrogen evolute.11 Hence, owing to these properties, 2D materials are used as environmental sensors. Based on these 2D materials, different sensing techniques are evolved for environment pollutant, sensing such as fluorescence, chemiluminescence, plasma mass spectrometry, electrochemical, and surface-enhanced Raman spectroscopy. The classification of layered materials based stability is shown in Figure 10.1.12

10.3 10.3.1

CLASSIFICATION OF TWO-DIMENSIONAL MATERIALS BASED ENVIRONMENTAL SENSORS grAphene

Graphene has been regarded as a marvel material since its discovery. This exceptional material is distinct because of its incredible optical, electrical, and magnetic properties. It was accidentally created in small amounts, hundreds of years before actual adventure, using pencil and other graphite-based materials on electron microscopes in 1962. The material was subsequently found and described in 2004 by Andre Geim and Konstantin Novoselov, and they were granted the Nobel Prize in Physics in 2010 for the same.13 Isolating high-quality graphene, which had previously seemed unachievable, was no longer so tough.

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Graphene consists of a thin layer of carbon atoms that are densely arranged by a covalent bond and sp2 hybridized. It is a carbon allotrope with a honeycomb lattice made up of a monolayer of atoms. The name graphene is an association of “graphite” and the suffx “-ene” demonstrates that the graphite allotrope of carbon comprises stacked graphene layers. Each atom in a graphene sheet is connected by a σ-bond to its three close neighbors and contributes one electron to a conduction band that extends over the whole sheet.14 As a result, it can be regarded as the integral part of other allotropes; for instance, fullerenes, graphite, and CNTs can be pictured as wrapped-up sheets of graphene, while graphite can be visualized as stacked-up graphite sheets with relatively low distances between them around 3.37 Å. Although these allotropes are not created from graphene, in reality it is plausible. Graphene is stable and, at room temperature, exhibits magnifcent electronic and mechanical properties like thermal conductivity of 5,000 W m−1 K−1, the electrical conductivity of up to 6,000 S cm−1, charge-carrier mobility of 250,000 cm2 V−1 s−1, and a large theoretical specifc surface area of 2,630 m2 g−1. Moreover, it is highly transparent, with a retention of 250°C and keeping it below their melting point, then MO behaves as semiconductive material. This property permits MO to be broadly carried out in the gas sensor advances. Customary gas sensors made of metal oxides, for example, generally work at 100–400°C, prompting high power consumption and decreased sensor lifetime and stability due to instigated development of grains in the metal oxide. The manufacturing cost of MO is low in comparison to other sensing devices.

10.3.5

2D METAL-ORGANIC FRAMEWORK

Metal-organic frameworks are fashioned via self-assembly of molecules of inorganic metallic nodes with organic ligands. First, the concept of MOF was reported by

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Yashi and his group.33 In 1995, 60% of porosity and 2,900 m2g−1 specifc surface area was reported by them34 on account of their properties and structure as high density of active sites, high surface area, high catalytic activity. Tailorable pore size MOF based sensor has been used for environmental contaminant detector together with gases, organic compounds, anions, and heavy metal ions. Electrochemical, optical, and feld-effect transistor sensors are MOF-based environment sensors. The MOFs can have a high Langmuir surface area (>10,000 m2g−1), which makes it highly adsorbent toward analytes.35 In water environments, heavy metals are one of the biodegradable pollutants. Even at trace levels, several heavy metal ions such as chromium, copper, cadmium, mercury, etc. are exceptionally harmful and risky to human health. Subsequently, a lot of MOF sensors for substantial metal particle identifcation in water have been reported.

10.3.6

MXENES BASED 2D MATERIALS

MXenes is another group of 2D materials that belong to the IV group. More than 80 MXnes have been theoretically predicted, and about 40 different MXene have been synthesized.36 MXenes consist of ternary nitrides and carbides having formula Mn−1AXn, where M denotes TM as Sc, V, Ta, Cr, etc. A belongs to group IIIA and group IVA elements, and X can either be C or N, where n  =  1, 2, and 3 to form MXenes. In 2011, the primary MXenes material Ti3C2 was synthesized using layered hexagonal ternary carbide, by specifcally etching the Al atoms in the layer, to form Ti3AlC2 with hydrofuoric acid at room temperature. MXenes have a layered structure that drives from MAX phases due to fast intercalation of various cations. The metallic conductivity of MXenes is up to 24,000 Scm−1 and has a hydrophilic surface, which make it a good sensing material. The CNTs integrated between MXenes nanosheets prevent self-restacking and a fast ion diffusion process. In addition to their surface terminations, electronic features of MXenes rely upon the nature of M, X. Hypothetical investigations demonstrated that, for specifc composition with energy gaps about 0.25–2.0 eV, MXenes might behave metallic to semiconductor, except some heavy transition metal (e.g., Cr, Mo) containing MXenes, which may behave as an insulator. The preparation method for MXenes infuences their electronic properties because of the termination and different extent of imperfections. MXenes possess intense absorption properties in UV-visible to NIR regions. MXene has excellent charge carrier mobility, making it an excellent medium for potential electrical interaction between the protein and the MXene based electrodes.

10.4

MONITORING OF ENVIRONMENTAL CONTAMINANTS

Various types of analytical tests are done to characterize the prepared sensor and for the knowledge of its performance. Some common tests are scanning electron microscopy (SEM), scanning transmission electron microscopy (STEM), cyclic voltammetry (CV), square wave voltammetry (SWV), electrochemical impedance spectroscopy (EIS), square-wave anodic stripping voltammetry (SWASV), X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), the transmission electron microscopy (TEM), thermogravimetric analysis (TG), UV-Vis

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spectroscopy, and differential pulse voltammetry (DPV). Different contaminants are described in the next section.

10.4.1

HEAVY METALS

Heavy metals like Pb, Ag, Au, Th, Co, Cr, Hg, Zn, etc. are used in construction of alloys, batteries, coating, electroplating, pigments, and so on. But pouring, processing, burning of fossil fuel, mining, inappropriate disposal, casting have made nonbiodegradable and extremely virulent heavy metals a threat to humans and environment.37 Cd2+ may be carcinogenic and causes proteinuria, lung cancer, osteomalacia; whereas increased amount of Pb2+ may cause kidney injury, neurobehavioral development, high blood pressure, and anemia.38 Excess consumption of Hg causes minamata, kidney and respiratory failure, attention defciency in children, pulmonary edema,39 cardio vascular disease, nephrotic syndrome, loss of IQ, cyanosis syndrome, etc.; whereas surplus amount of As3+ in water may lead to black foot and palm, cancer of bladder and mutations.40 Cu2+, Zn2+ are also toxic enough. Normal ways of detecting heavy metal ions are X-ray fuorescence, inductively coupled plasma mass spectroscopy, atomic absorption spectroscopy (AAS), inductively coupled plasma atomic emission spectroscopy. However these are lab-based techniques and cannot be used for on-site analysis. MoS2 flm, which is decorated with DNA functionalized Au Nps, can potentially serve as an FET sensor. The formation of which is shown in Figure 10.2.22 Hg detection can be done using it with LOD lower than 0.1 nM and

FIGURE10.2 (a) Schematic diagram for the fabrication process of the MoS2/DNA-Au NPs hybrid structure. (b) FET sensor platform based on the hybrid structure.22

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response time as low as 1–2 s. The detector even provides benefts like low price and high selectivity against other metal ions and acceptable real-time analysis.22 Another FET device can be obtained by using polydimethylsiloxane coupled with MoS2, which offers detection limit of 30 pM for Hg2+.41 Organic Hg is one of the most threatening segments of them. Due to its lipophilic nature, CH3Hg causes several damages to the human body. Aldehyde fabricating industry is one of the reasons of formation of CH3Hg. 3D element doped haeckelite BN based sensor was reported for CH3Hg sensor. Milks is a staple food all over the world, but it may contain various amounts of toxic heavy metals. A sensing electrode is reported to detect these kinds of heavy metals in milk. The electrochemical transducer used a novel bismuth flm electrode based on electrochemically reduced graphene oxide (ERGNO) modifed with SPE via electrodeposition method. It was used to detect Cd2+ and Pb2+ and shows a linear relationship in concentration of 1–60 μg L−1 with peak current. LOD for Cd2+ was 0.5 μg L−1 and for Pb2+ was 0.8 μg L−1. Deposition potential and deposition time for both the metal were −1.2 V and 150 s, respectively.42

10.4.2

ORGANIC COMPOUNDS

Phenolic compounds are extensively used in various sectors, like pharmaceuticals, pesticides productions, wood preservation, petrochemical manufacturing, etc.43 Due to its toxic nature, phenolic compounds have become a threat to humans and environment. A Pd-graphene/1-Butyl-3-methylimidazolium tetrafuoroborate/nafon composite electrode can detect 2-chlorophenol(2-Cp). Mixture of IL and sonochemically prepared Pd-graphene was dropped on polished GCE. Then the composite was dried, and again nafon was dropped. Detection range was 4–800 μmol L−1, and detection limit was 1.5 μmol L−1. Electrode shows excellent reproducibility, but 4-Cp interferes the peak current.44 A CuO/nafon/GCE can be prepared by dropping mixture of nafon and CuO synthesized through aqueous chemical growth, on polished GCE. Figure 10.3 shows Ipa response increases linearly with increasing concentration of 2, 4, 6 Trichlorophenol LOD, and LOQ was found to be 0.046 and 0.154 μM, respectively.45 4-nitrophenol (4-Np) is one of the pollutants found in the environment and is extremely hazardous. A GCE modifed by polyvinylpyrrolidone (PVP) assisted ultrasonically by exfoliated BP can potentially detect trace amount of 4-Np. The electrode shows DPV oxidation response at 1.09V for 20 μM. The oxidation peak current shows linearity with concentration in a range 0.10–5.0 μM, giving LOD 0.028μM. But the peak current decreases slightly in presence of similar compounds.46 Presence of explosive or explosive-like substances is a big question to the homeland security and environmental safety. Thus, in this concern, J. Wu et al. manufactured TNT-specifc peptide functionalized AuNPs/MoS2 sensor. AuNPs/MoS2 was prepared through one step conjugation and functionalized with TNT-specifc peptide designed with sulfydryl group. TNT and peptide molecule manifested charge transfer interaction and thus optical spectra. Addition of TNT shifts the fundamental absorption peak from 541 nm. Upon increasing concentration of TNT from 2 × 10−7 to 1 × 10−14, M absorption peak value decreases linearly. The sensor is highly selective to TNT, but DNT shows slight response.47

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FIGURE 10.3 Schematic diagram of formation of CuO/nafon/GCE electrode and Ipa response curve.45

TABLE 10.1 Other Examples of Organic Molecule Detection Using 2D Materials Organic Molecule

Sensor

Detection Range

Hydroquinone Catacol Nitromethane Phenol Nitrobanzene Tartrazine

Ti3C2-MWCNTs/GCE

2 μM to 150 μM

Hb-CS/rGO-CS m-GMN/c-PGEs Cd MOF g-C3N4/PGE

Sunset yellow

GCE/ERGO

5 μM to1.46 mM 1 µg/mL to 7 µg/mL 0.010 mM to 0.060 mM 1.0 × 10−7 to 1.0 × 10−5 mol L−1 50–1,000 nM

10.4.3

LOD

References

6.6 nM 3.9 nM 1.5 μM 0.35 µg/mL 0.1 μM 0.21 μmol L−1 19.2 nM

48 49 50 51 52 53

PESTICIDES

Pesticides are the second largest man-made chemicals after fertilizers.54 Pesticides are chemicals that have agricultural applications in controlling or eliminating pests and unwanted weeds. Five major classifcations of pesticides according to their composition are carbamate, organophosphate, pyrethroids, pyrethrin, and organochlorine. Abuse, inappropriate handling, and accidental release of these may cause

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serious venture to humans as well as environment.55 Gas Chromatography (GC), gas chromatography/mass spectrometry (GC/MS), fuorometry, liquid chromatography/ mass spectrometry (LC-MS), high-performance liquid chromatography (HPLC), mass spectrometry (MS) are the traditional ways of determining pesticides.56 But extreme complicated instrumentation, pre- and post-treatment of sample, requirement of trained staff, high analytical cost, large time requirement restricts the real-time application, especially in emergency. In agricultural production, organophosphorus pesticides (OPs) are extensively used. The most poisonous OP is called paraoxon ethyl (PXL), which may cause poor visualization, diarrhea, hunger edema, tumor, neurotoxic effect, acute eye, even death.57 PXL can be detected by using Pd NPs modifed boron nitride (BN) heterojunction. First Pd NPs/BN HJ was fabricated by sonochemical method and further drop casted on glassy carbon electrode. Pd NPs/BN HJ provides working range from 0.09 to 210 μM and LOD 0.05 μM.58 Other Ops, methyl parathion (MP), diazinon (DIA), and chlorpyrifos (CHL), in water sample can be determined by using BN quantum dot decorated GO obtained through ultrasonication and further drop casting of them on GCE. Linearity range was found to be 1.0  ×  10−12− 1.0  ×  10−8 M, while LOD of MP, DIA, CHL are 3.1  ×  10−13, 6.7 × 10−14, and 3.3 × 10−14 M, respectively. Good reproducibility, sensitivity, and stability were obtained for the sensor in water and apple juice as well.59 A nanocomposite of Prussian blue (PB) functionalized on Ti3C2Tx was prepared by reduction of Fe(III) and Fe(CN)63− mixture with Ti3C2Tx. Then acetylcholinesterase (AchE) was immobilized on a PB@ Ti3C2Tx modifed GCE. It can potentially detect malathion in the range 0.0001–1.0 nM and LOD 1.3 × 10−16 M.60 A nonenzymatic sensor was prepared for the detection of nitroaromatic OPs. First, silicon passivation was done on a silver screen printed electrode through roll-to-roll slot die coating method. Then GNPs were air brushed on the electrode in a roll-to-roll process. Last, ZrO2 were electrosprayed in roll-to-roll process. The fabricated electrode showed rapid response time (10 min), high stability of 89.4% after 30 days, extreme sensitivity of 0.2 ppm with LOD 1 μM for methyl parathion.61

10.4.4

ANTIBIOTIC COMPOUNDS

Due to the nondegradable characteristics of antibiotics in drinking water, food products, and environment, it may cause many hazardous effects like disturbance in ecological equilibrium, allergic reactions, antibiotic-resistant bacterial strain, etc. Traditional analytical methods to detect antibiotics are capillary electrophoresis, surface enhanced Raman scattering (SERS), HPLC coupled with UV, LC-MS, supercritical fuid chromatography/mass spectrometry (SFC-MS), spectrophotometry, fow injection chemiluminescence (FI-CL), capillary zone electrophoresis (CZE), thinlayer chromatography, etc.62,63 But these techniques are inconvenient as it requires expert staff for handling, long time of detection, complicated process. Some antibiotics that are reported to be detected simply using 2D materials are chloramphenicol, linezolid, kanamycin, ornidazole, nitrofuran, sulfamethoxazole.64–66 Penicillin is a β-lactam based multipurpose antibiotic that is used in preventing and controlling various bacterial diseases in humans and animals. But relatively 5–10% population suffers from hypersensitivity of penicillin, causing allergic reaction for ultralow

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concentration as 1 ppb.67 Y. Wu et al. constructed an ultrasensitive penicillin sensor using layer-by-layer method. First, single graphene nanosheet was prepared reducing GO (synthesized via Hummer’s method) with NaBH4. Further dispersed mixture of GO and hematein was drop casted on polished GCE. Next trihexyltetradecylphosphonium bis (trifuoromethylsulfonyl) imide (IL) and penicillinase were immobilized on the modifed GCE one by one. Detection range was 1.25 × 10−13− 7.5 × 10−3 M with detection limit 10−13 M (0.04 ppt). Good storability and selectivity were offered by the electrode with successful real-time sample detection.68 Metronidazole (MNZ) is a widely used antibiotic that used in treatment of nosema apis,69 infections, and acne diseases. But accumulation of MNZ is proven to be mutagenic and carcinogenic to humans. For the detection of MNZ, a molecular imprinted polyimide (MIP)/2D Sn3O4 was prepared. Sn3O4 was developed via hydrothermal method, above which polypyrrole particles were formed by chemical oxidative polymerization. The DPV peak current was linear with concentration of MNZ in the range 0.025–2.5 μM with LOD as low as 0.0032 μM. The sensor shows good selectivity, stability (92% after three weeks in room temperature) and practicability.70 A popular drug used against gram positive bacteria is midecamycin (MD). A GO drop casted on GCE can potentially detect MD with detection range 3 × 10−7− 2 × 10− 4 mol L−1 with LOD 1 × 10−7 mol L−1.71 A lanthanum vanadate/functionalized boron nitride (LaV/BN) nanocomposite were prepared by sonochemical method for the detection of furazolidone (FZD). Figure 10.4 shows procedure of formation of LaV/BN/GCE electrode and CV signal of LaV/BN/GCE in different concentrations of FZD.72 The composite was added on GCE by dropping. Peak current increases with increasing concentration of FZD from 50 to 250 μM, where the detection limit is 0.003 μM. Acceptable stability and real-time sample analysis is offered by this electrode.72

FIGURE 10.4 Procedure of formation of LaV/BN/GCE electrode and CV signal of LaV/BN/ GCE in different concentrations of FZD.72

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BACTERIA

Few foodborne bacteria are Salmonella typhimurium, Escherichia coli O157:H7, Staphylococcus aureus, Listeria monocytogenes, etc.73 Detection of bacteria is normally done by processes that use culture techniques. But bacteria culture techniques are very much time engrossing and have very low positive rate (50–60%). New different ways are proposed like enzyme-linked immunosorbent assays (ELISA), polymerase chain reaction (PCR), nucleic acid amplifcation tests (NAATs).74–76 But to perform ELISA, one requires enzyme-supporting antibodies while PCR is time-consuming, whereas for NAATs, highly equipped laboratory and skilled personnel are required. 2D materials are mainly functionalized with aptamers or antibodies so that they can bind with the bacteria.77 A new idea to identify E. coli UT189 was proposed by using amine group functionalized PEI (polyethyleneimine) modifed rGO electrode, which was further modifed by anti-fmbrial antibody E. coli antibodies along with PEG (polyethylene glycol) modifed pyrene units. LOD was 10 cfu mL−1, and only 2.8% peak current was lost upon storage in PBS buffer at 4°C for two weeks78 Figure 10.5 shows a holey reduced GO (hRGO) FET sensor that was functionalized with an antimicrobial peptide, magainin I that can potentially sense gram-negative bacteria.79 Through reduction in the conductance, the device gives signal of the presence of bacteria. LOD was found to be 803 cfu mL−1 but the device is not reusable.79

FIGURE 10.5 (a) Schematic of bacteria binding on a 2D sensor, (b) and (c) conductance (G) versus gate voltage (Vg) of bare hRGO FET devices in presence of E. coli and Listeria.79

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E. coli causes most of the diseases like diarrhea, pneumonia, urinary tract infections, etc. An aminoterepthalic acid functioned GO based sensor was derived for E. coli detection. Aminoterepthalic acid adds amine functional group on the GO. The produced sensor shows good reproducibility and stability (up to 60 days) with LOD 2 cfu mL−1 and four-minute response time.80 Mycobacterium tuberculosis (MT) is one the common bacteria that causes tuberculosis. But recently they are reported to be drug resistant and often mutate their genes.

10.4.6

GASES

Gas detection is important in sectors like industrial productions, indoor air quality, greenhouse gas detection, and automobile exhaust measurements. Current commercial gas sensors are majorly based on metal oxide semiconductors, which works in elevated temperature. But to generate high temperature, the device needs high power and application of them in portable device. Nitrogen dioxide (NO2) is one of the most toxic and death-dealing gas, which is harmful to humans over 1 ppm concentration.81 NO2 is one of the gases that majorly is produced in various industries and vehicles. It is a perilous and death-dealing gas that may cause numerous health problems. It may cause respiratory diseases like emphysema, bronchitis, and cardiovascular diseases. It has other harmful effects, like acid rain, primary cause of photochemical smog. It is formed as a byproduct of internal combustion engines, food processing, metal and petrol refnery, and thermal power stations. NO2 is also used as biomarkers in diagnostic processes like infection of lungs tissue and gastrointestinal disorder symptoms.82 A composite was prepared by using SnS2 decorated rGO, which shows increased gas sensing ability in relatively lower temperature. The composite was tested for various gases (NO2, NH3, CH4, C2H5OH, H2O, and acetone). But the flm shows no response for CH4, C2H5OH, and acetone. NH3 shows 15.3% response for 99 ppm only above 80°C. At 25°C and 50°C 11.9 ppm, NO2 shows an irreversible ~85% and ~78% response, respectively. At 80°C, a 56.8% reversible response is obtained upon 5 minute exposure to 11.9 ppm NO2, and minimum ~2.8 minute response time is required.83 Due to the strong interaction of NO2 and sp2 carbons, graphene and CNTs show unsatisfactory high response time despite having great response. But 2D chalcogenides may be a better option. It was proved that suspended BP can give much better reproducibility and gas response than supported BP (23% increased at 200 ppm).84 Multilayer BP was utilized in preparation of a feld-effect transistor (FET) for NO2 gas detection. It owns magnifcent selectivity of 5 ppb and response time of ~280–350 s for different concentrations.85 H. Cui et al. prepared a tellurene gas sensor, which can detect NO2 in the range from 25–150 ppb in dry air at room temperature.86 This sensor provides LOD as low as 0.214 ppb and even stable till 100°C. Response and recovery time was found to be 83 s and 485 s for 25 ppb and 26 s and 240 s for 100 ppb NO2.86 Humidity sensing is important in food industry, health care, weather forecasting, and chemical industry.87 For the frst time TiSi2 was tested as humidity sensing materials. Integrated electrode was formed by depositing 2D nanofakes of TiSi2. New sensor shows impedance sensitivity of 63 KΩ% RH at 1 KHz. It shows good reproducibility and detection range from 0–100% relative humidity with stability up to 360 minutes.88 An ammonia sensor electrode was prepared with scattered ZnO on silver paste, which was

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layered on an electrode. With concentration of NH3 less than 2 ppm, the sensor provides reversible signal, which decreases slightly with increasing temperature from 250°C to 300°C. It can detect in the range of 600 ppb to 3 ppm NH3 at 250°C. Rapid maximum response of 80% was obtained with good recovery. But baseline drift was observed, which was manipulated by baseline manipulation method.89

10.4.7

VOLATILE ORGANIC COMPOUNDS

Volatile organic compounds (VOCs) are organic compounds with low boiling point and high vapor pressure. These are common air pollutant that forms many carcinogenic compounds, thus harmful to human health.90 Xylene may cause pneumonia, poisoning, dermatitis, whereas formaldehyde causes sick building syndrome.91 VOCs can be detected via many sophisticated analytical techniques like fuorescence spectroscopy, fow-tube mass spectrometry, optical spectrometry, surface acoustic wave sensing, GC-MS, and quartz crystal microbalance techniques.92 But these are inconvenient and time-consuming. An alternative is use of 2D materials in sensing devices, which can give fast results and in relatively low-cost and time. When Au decorated chemically exfoliated MoS2 was exposed to various VOCs, Figure 10.6

FIGURE 10.6 (a) Preparation of the Au-doped MoS2 gas sensor, (b) real-time resistance, and (c) normalized response (ΔR/Rb) of the pristine MoS2 and Au-doped MoS2 sensors exposed to various VOCs.93

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TABLE 10.2 Other Examples of VOC Detections VOCs xylene toluene acetone ethanol isopropanol glucose formaldehyde

2D Materials NiCo2O4 hollow microtubules CuO/Ti3C2TxMXene Ti3C2Tx WO3·0.33H2O NiO/GNS (graphene nanosheet) Au/In2O3

Detection Range

LOD

References

100 ppm 50 ppm 50–1,000 ppb 100–1,000 ppb 100 ppm 5 μM–4.2 mM 50 ppm

1 ppm 0.32 ppm 0.011 ppb 0.13 ppb 1 ppm 5.0 μM 1.42 ppb

95 96 97 98 99 100

shows how toluene, hexane gives positive response, whereas ethanol, acetone gives negative response.93 But in the case of aldehyde, response depends on the doping ratio of Au and MoS2.93 An inkjet-printed titanium carbide Ti3C2TxMxene/WSe2 (Tungsten diselenide) composite was designed by Ti3C2Tx nanosheet and WSe2 nanofakes formed by liquid phase exfoliation. EtOH response was found to decrease with increasing amount of WSe2 and increasing flm thickness. It takes only 9.7 s to respond toward EtOH and 6.6 s for recovery at room temperature. The sensor is quite stable in low humidity that it offers 92% of its initial response after ten days in dry condition. It shows maximum response for EtOH and then MeOH.94

10.5 FUTURE PERSPECTIVE AND CONCLUSIONS 2D materials show exclusive interesting properties like enhanced surface area, absorption property. But they also have several problems. Practically, graphene is inert in nature and thus needs other treatment to activate it, which may lead to degradation of some of its properties. BP is also not really stable in environment. MoS2 being a typical sulfde, its stability in aqueous media can be questioned, and it may oxidize easily in the presence of some oxidizing agent leading to formation of harmful ion like SO42−.101 These challenges need to be overcome. Though MoS2 and WSe2 are explored extensively, other examples of 2D chalcogenides are very rare. Environment stability of BP can be enhanced by surface functionalization or polymer coating.102 But these ways need to be explored. Hybridization of 2D materials with other compounds imparts a great success, and there are plenty of examples of sensors for detection of various compounds. But very few of them are practically possible. Preparation of these sensor in commercial scale is a big challenge. Except all these improvement in LOD, selectivity, sensitivity, long-term usability, cost is essential. Majorly, LOD can be archived in ppb range with 2D materials. Though there are few examples of ppt range too. Some sensors require high temperature, specifcally gas sensors. It required further demand to room temperature gas sensors. Some vital sensors are also not reusable due to the strong interaction with the analyte. Biofouling—that is, unwanted growth of microorganisms, algae, plants, etc. on sensor surface, which affects its performance, in presence of moisture—is also a challenge that requires attention. By polymer coating, it can be controlled to some

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extent. Last, understanding of analyte sensor interaction and sensing mechanism is very much important and needs to be fully known as only this can be the key to open the benefts of 2D materials completely.

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78. Jijie, R.; Kahlouche, K.; Barras, A.; Yamakawa, N.; Bouckaert, J.; Gharbi, T.; Szunerits, S.; Boukherroub, R., Reduced graphene oxide/polyethylenimine based immunosensor for the selective and sensitive electrochemical detection of uropathogenic Escherichia coli. Sensors and Actuators B: Chemical 2018,260, 255–263. 79. Chen, Y.; Michael, Z. P.; Kotchey, G. P.; Zhao, Y.; Star, A., Electronic detection of bacteria using holey reduced graphene oxide. ACS Applied Materials & Interfaces 2014,6 (6), 3805–3810. 80. Gupta, A.; Bhardwaj, S. K.; Sharma, A. L.; Deep, A., A graphene electrode functionalized with aminoterephthalic acid for impedimetric immunosensing of Escherichia coli. Microchimica Acta 2019,186 (12), 800. 81. Novikov, S.; Lebedeva, N.; Satrapinski, A.; Walden, J.; Davydov, V.; Lebedev, A., Graphene based sensor for environmental monitoring of NO2. Sensors and Actuators B: Chemical 2016,236, 1054–1060. 82. Ou, J. Z.; Yao, C. K.; Rotbart, A.; Muir, J. G.; Gibson, P. R.; Kalantar-zadeh, K., Human intestinal gas measurement systems: in vitro fermentation and gas capsules. Trends in Biotechnology 2015,33 (4), 208–213. 83. Shafei, M.; Bradford, J.; Khan, H.; Piloto, C.; Wlodarski, W.; Li, Y.; Motta, N., Lowoperating temperature NO2 gas sensors based on hybrid two-dimensional SnS2-reduced graphene oxide. Applied Surface Science 2018,462, 330–336. 84. Lee, G.; Kim, S.; Jung, S.; Jang, S.; Kim, J. H., Suspended black phosphorus nanosheet gas sensors. Sensors and Actuators B-Chemical 2017,250, 569–573. 85. Abbas, A. N.; Liu, B.; Chen, L.; Ma, Y.; Cong, S.; Aroonyadet, N.; Köpf, M.; Nilges, T.; Zhou, C., Black phosphorus gas sensors. ACS Nano 2015,9 (5), 5618–5624. 86. Cui, H.; Zheng, K.; Xie, Z.; Yu, J.; Zhu, X.; Ren, H.; Wang, Z.; Zhang, F.; Li, X.; Tao, L.-Q.; Zhang, H.; Chen, X., Tellurene nanofake-based NO2 sensors with superior sensitivity and a sub-parts-per-billion detection limit. ACS Applied Materials & Interfaces 2020,12 (42), 47704–47713. 87. He, J.; Xiao, P.; Shi, J.; Liang, Y.; Lu, W.; Chen, Y.; Wang, W.; Théato, P.; Kuo, S.-W.; Chen, T., High performance humidity fuctuation sensor for wearable devices via a bioinspired atomic-precise tunable graphene-polymer heterogeneous sensing junction. Chemistry of Materials 2018,30 (13), 4343–4354. 88. Shaukat, R. A.; Khan, M. U.; Saqib, Q. M.; Chougale, M. Y.; Kim, J.; Bae, J., All range highly linear and sensitive humidity sensor based on 2D material TiSi2 for real-time monitoring. Sensors and Actuators B: Chemical 2021,345, 130371. 89. Kanaparthi, S.; Govind Singh, S., Highly sensitive and ultra-fast responsive ammonia gas sensor based on 2D ZnO nanofakes. Materials Science for Energy Technologies 2020,3, 91–96. 90. Kampa, M.; Castanas, E., Human health effects of air pollution. Environmental Pollution 2008,151 (2), 362–7. 91. Akiyama, T.; Ishikawa, Y.; Hara, K., Xylene sensor using double-layered thin flm and Ni-deposited porous alumina. Sensors and Actuators B: Chemical 2013,181, 348–352. 92. Buszewski, B.; Ulanowska, A.; Ligor, T.; Denderz, N.; Amann, A., Analysis of exhaled breath from smokers, passive smokers and non-smokers by solid-phase microextraction gas chromatography/mass spectrometry. Biomedical chromatography : BMC 2009,23, 551–6. 93. Cho, S.-Y.; Koh, H.-J.; Yoo, H.-W.; Kim, J.-S.; Jung, H.-T., Tunable volatile-organiccompound sensor by using Au nanoparticle incorporation on MoS2. ACS Sensors 2017,2 (1), 183–189.

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94. Chen, W. Y.; Jiang, X.; Lai, S.-N.; Peroulis, D.; Stanciu, L., Nanohybrids of a MXene and transition metal dichalcogenide for selective detection of volatile organic compounds. Nature Communications 2020,11 (1), 1302. 95. Du, L.; Song, X.; Liang, X.; Liu, Y.; Zhang, M., Formation of NiCo2O4 hierarchical tubular nanostructures for enhanced xylene sensing properties. Applied Surface Science 2020,526, 146706. 96. Hermawan, A.; Zhang, B.; Taufk, A.; Asakura, Y.; Hasegawa, T.; Zhu, J.; Shi, P.; Yin, S., CuO nanoparticles/Ti3C2Tx MXene hybrid nanocomposites for detection of toluene gas. ACS Applied Nano Materials 2020,3 (5), 4755–4766. 97. Kim, S. J.; Koh, H.-J.; Ren, C. E.; Kwon, O.; Maleski, K.; Cho, S.-Y.; Anasori, B.; Kim, C.-K.; Choi, Y.-K.; Kim, J.; Gogotsi, Y.; Jung, H.-T., Metallic Ti3C2Tx MXene gas sensors with ultrahigh signal-to-noise ratio. ACS Nano 2018,12 (2), 986–993. 98. Perfecto, T. M.; Zito, C. A.; Mazon, T.; Volanti, D. P., Flexible room-temperature volatile organic compound sensors based on reduced graphene oxide—WO3·0.33H2O nano-needles. Journal of Materials Chemistry C 2018,6 (11), 2822–2829. 99. Zeng, G.; Li, W.; Ci, S.; Jia, J.; Wen, Z., Highly dispersed NiO nanoparticles decorating graphene nanosheets for non-enzymatic glucose sensor and biofuel cell. Scientifc Reports 2016,6 (1), 36454. 100. Gu, F.; Di, M.; Han, D.; Hong, S.; Wang, Z., Atomically dispersed Au on In2O3 nanosheets for highly sensitive and selective detection of formaldehyde. ACS Sensors 2020,5 (8), 2611–2619. 101. Sabaraya, I. V.; Shin, H.; Li, X.; Hoq, R.; Incorvia, J. A. C.; Kirisits, M. J.; Saleh, N. B., Role of electrostatics in the heterogeneous interaction of two-dimensional engineered MoS(2) Nanosheets and natural clay colloids: infuence of ph and natural organic matter. Environmental Science & Technology 2021,55 (2), 919–929. 102. Xu, Y.; Wang, W.; Ge, Y.; Guo, H.; Zhang, X.; Chen, S.; Deng, Y.; Lu, Z.; Zhang, H., Quantum dots: stabilization of black phosphorous quantum dots in PMMA nanofber flm and broadband nonlinear optics and ultrafast photonics application (Adv. Funct. Mater. 32/2017). Advanced Functional Materials 2017,27 (32).

11

2D Nanomaterials in Diagnostics and Therapy of Cardiovascular Diseases Pooja Yadav, Samir K. Beura, Abhishek R. Panigrahi, Abhinaba Chatterjee, Jyoti Yadav, and Sunil K. Singh

CONTENTS 11.1 11.2

Introduction..................................................................................................217 2D Nanomaterials ........................................................................................219 11.2.1 Graphene.........................................................................................219 11.2.2 MoS2 ...............................................................................................219 11.2.3 Other 2D Nanomaterials ............................................................... 220 11.2.3.1 TMD .............................................................................. 220 11.2.3.2 BN-Nanosheet................................................................ 220 11.2.3.3 g-C3N4 Nanosheet ...........................................................221 11.2.3.4 Black Phosphorus............................................................221 11.3 Cardiovascular Diseases (CVDs) .................................................................221 11.3.1 Cardiovascular Biomarkers as Diagnostic Parameters ................. 222 11.4 2D Nanomaterials in Diagnostics ............................................................... 227 11.4.1 Graphene........................................................................................ 227 11.4.2 MoS2 .............................................................................................. 228 11.4.3 Other 2D Nanomaterials ............................................................... 228 11.5 Therapeutic Application of 2D Nanomaterials ........................................... 229 11.6 Challenges and Future Perspective ..............................................................232 11.7 Conclusions ..................................................................................................232 References...............................................................................................................233

11.1 INTRODUCTION With the era of advancement in nanotechnology, 2D nanomaterials have attracted ample interest for next-generation materials. The focus on 2D material starts with the discovery of graphene sheet1 and advances with the mechanical cleavage of graphene from graphite.2 Since then, other 2D nanomaterials such as transition metal dichalcogenides (TMDs), black phosphorus nanosheets, boron nitride nanosheet (BNNS), DOI: 10.1201/9781003247890-11

217

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metal-organic framework (MOF) nanosheets, and others have been investigated to be applied in various felds.3 The synthesis of 2D nanomaterials can be carried out by two major approaches, including top-down or bottom-up approaches.4 2D nanomaterials display a wide range of promising applications including electronics,5 energy storage (e.g., wearable batteries, supercapacitors),6 catalysis,7, sensors as well as biomedicine.8 Unique properties of 2D materials such as ultralow weight, high strength, high reactivity,9 optical properties (like polarized photoluminescence),10 plasmonic behavior,11 unique physicochemical responsiveness, layer-dependent band structure,12 ability to fabricate13 make them ideal agents to be employed for diagnostics as well as therapeutic purpose in the biomedical feld. Several reports are available to embark the 2D nanomaterials in health care applications, such as tissue engineering, regenerative nanomedicines,14 bone therapy,15 cancer therapy,3 neuronal regeneration,16 diabetic detection,17 and wound healing.18 In this chapter, we review the theranostic potential of 2D nanomaterials in cardiovascular diseases (Figure 11.1). Cardiovascular diseases (CVDs) impose serious health concerns among the global community as they are the widespread reason for death in developing as well as developed countries.19 Cardiomyopathy refers to the complex group of heart diseases, where heart muscle becomes damaged, and further replaced with noncontractile scar tissue, thus making it unable to pump enough blood throughout the body to maintain a normal functioning of a system. This could lead to the heart failure (HF) and ultimately death of a person.20 The survival of patient in cardiomyopathy requires early diagnosis and appropriate treatments. The major limitation associated with the diagnosis process is that most of the cardiac-related complications exhibit similar symptoms. It introduces the need for cardiac biomarker detection in this feld, which exhibits its specifcity to a particular disease among the cocktail of cardiac disorders. High specifcity, increased sensitivity, quick release, and quantitative assay mark the ideal characteristics for a cardiac biomarker.21 Increased cardiac biomarker concentration in serum exhibits a strong correlation with recurrent cardiovascular complications; hence the detection of biomarkers with the appropriate device also imparts a critical role in successful treatments of the patient. In recent years,

FIGURE 11.1

2D nanomaterials in diagnostics and therapeutics of cardiovascular disease.

* BNP: brain natrieuretic hormone; PMV: platelet microvesicle; MoS2: molybdenum disulfde; TMD: transition metal dichalcogenide

2D Nanomaterials, Cardiovascular Diseases

219

biosensing emerges as the most promising platform in the early detection of cardiac marker.22,23,24 In addition to diagnosis, treatment strategies also imbibe its crucial role in the survival of patients, which includes tissue repair and regeneration program. Incorporation of nanomaterials in creating a biocompatible scaffold to replace the heart’s scar tissue has gained signifcant interest in current era.25 In this chapter, we explore the cardiac disease association with 2D nanomaterials in terms of diagnostics as well as therapeutics. In summary, we explain general 2D nanomaterials like graphene, MoS2, and others with their signifcant contribution in biosensing of cardiac biomarkers as well as their therapeutic potential implied in tissue repair, which occurs during cardiac diseases (Figure 11.1).

11.2

2D NANOMATERIALS

2D nanomaterials are ultrathin materials that are single or few atomic layers thick in one of their dimensions, while the rest two dimensions are outside the 100 nm range. In sheetlike 2D nanomaterials, their transverse dimensions are larger than 100 nm, whereas their thickness is usually measured to be less than 5 nm. The unique shape of 2D material provides broad surface area and anisotropic physical/chemical properties to it.26 Based on their synthetic strategies, 2D materials are categorized under top-down and bottom-up approaches. In top-down approach, the nanoscale materials are created by removal of materials from its bulk size in a controlled manner, whereas bottom-up approach uses atomic or molecular precursors and permits them to self-assemble for the development of more complex structures.4

11.2.1

GRAPHENE

Graphene is an allotrope of carbon arranged in two-dimensional (2D) honeycomb sheetlike structure. Graphene is also considered to be the thinnest material having higher mechanical strength than steel.27,28 Generally, graphene nanosheets are generated by two conventional approaches, that is, top-down (by carbon nanotube (CNT) splitting and/or chemical/mechanical exfoliation of graphite) and bottom-up (by total organic synthesis and/or chemical vapor deposition from precursor molecules).28,29,30 Furthermore, graphene has some unique physical and mechanical properties, including excellent electrical conductivity because of its sp2 hybridized orbitals (108 S/m) and high thermal conductivity (5,000 W/mK).31,32 Apart from these, it also possesses an enlarged specifc area, strong mechanical strength, and an easily modifable (chemically) surface (e.g., carboxylation, polyethylene glycation, etc.). Optically, graphene can produce gatedependent transition and can potentially display photoluminescence.33,34 Hence, considering all these characteristic properties, graphene is an obvious choice for being a widely used nanomaterial in diagnostic and theranostic applications.

11.2.2

MOS2

Molybdenum disulfde (MoS2) comes under a large family of compounds known as transition metal dichalcogenides (TMD). It is the second most widely studied 2D nanomaterial after graphene for its plausible applications, such as drug delivery

220

Emerging 2D Materials and Applications

and biosensors designing. In general, chemical vapor deposition31 or atomic layer deposition35 are used for the synthesis of MoS2. The crystalline structure of MoS2 is arranged in such a way that a layer of Mo atom is sandwiched between two layers of S atoms. With carrier mobility of 200 cm2/V/S.36 MoS2 has an advantage over graphene by forming a direct bandgap, which makes it a preferable choice for feld-effect transistors (FET). Like graphene, MoS2 is also considered a potential candidate for biosensor designing due to its salient properties, including photoluminescence, tunable energy bandgap, intercalated morphology, and liquid media stability.37,38

11.2.3

OTHER 2D NANOMATERIALS

11.2.3.1 TMD Transition metal dichalcogenides (TMDs) provide a promising alternative to graphene in the feld of 2D nanomaterials. TMDs are made up of one atom of transition metal (e.g., molybdenum (Mo), tungsten (W), etc.), which combines with two atoms of chalcogen (e.g., sulfur (S), selenium (Se), the metalloid tellurium (Te), etc.) making a generalized molecular formula MX2. Atomic-scale thickness, direct bandgap, favorable electronic and mechanical properties make researchers enthusiastic about their possible application in the biomedical feld.39 Synthesis of TMD involves chemical vapor deposition, mechanical exfoliation, heterostructure restructuring, and topochemical intercalation.40 In a study, Wang et al. were the frst to develop a horseradish peroxidase HRP/MoS2 based glassy carbon electrode (GCE) to detect an electrocatalytic reduction of H2O2 when ubiquinone was used as a mediator.41 Tungsten disulfde (WS2) based fuorescent biosensor for detection of T4 polynucleotide kinase (PNK) activity was also prepared due to excellent optical property exhibited by TMDs.42 In another report, Liu et al. developed a composite carrier (MoS2-PEG) by functionalizing MoS2 nanosheets with lipoic acid-modifed polyethylene glycol (PEG) to be utilized as a multifunctional drug carrier for combined cancer therapy.43 Thus, when it comes to the application of graphene-like nanomaterials, attempts to exploit the properties of TMD obviously draw the attention of researchers. 11.2.3.2 BN-Nanosheet Boron nitride (BN) nanosheets are formed by the alternative arrangement of boron (B) and nitrogen (N) atoms in a honeycomb lattice structures, which are isoelectric to carbon. These hexagonal BN nanosheets possess stronger mechanical strength with Young’s modulus about 0.865 TPa as compared to graphene. Furthermore, the mechanical strength of BN nanosheets decreases less dramatically when the thickness is increased.44 Other than mechanical characteristics, better surface adsorption, wide bandgap, thermal stability,45 better oxidation resistance,46 and excellent dielectric properties trigger interest among scientists. BN nanosheets are also excellent proton conductors.26 BN nanosheets can be synthesized by chemical vapor deposition,47 mechanical cleavage, solvent exfoliation,47 solid-state reaction,4 and sonication.48 Apart from these, multifunctional BN nanosheet-AuNP (gold nanoparticle) nanocomposite based immunosensors have been developed to detect

2D Nanomaterials, Cardiovascular Diseases

221

IL-6.49 Moreover, hexagonal boron nitride (h-BN) graphene quantum dot exhibits strong fuorescence property, and for being very low cytotoxic, this can also be used in cell imaging.50 11.2.3.3 g-C3N4 Nanosheet Graphitic carbon nitride (g-C3N4) is a nanopolymer composed of tris-triazine-based patterns with the carbon (C) and nitrogen (N) along with a small amount of H. g-C3N4 has electron-rich properties apart from H-bonding motifs, which makes it a potential candidate in the application of biomaterial science.51 g-C3N4 nanosheet shows an excellent thermal conductivity, better chemical stability with a bandgap of ca. 2.7 eV, and inherent photoluminescence (PL) for its unusual structure.52 Synthesis of g-C3N4 is a challenging task because of its low thermodynamic stability.53 Recently, ultrarapid synthesis of the polymer has been reported using direct microwave heating for feld emission.54 Zhang et al. developed graphene carbon nitride and manganese oxide (C3N4-MnO2) sandwich nanocomposite for the detection of glutathione,55 thus indicating possible biosensing application of this material in near future. 11.2.3.4 Black Phosphorus Black phosphorus, otherwise known as phosphorene, exhibits extraordinary electrical conductivity and electron transfer capability. In phosphorene, each phosphorus atom is covalently bound with three other phosphorus atoms forming a bilayer structure.56 Layer-dependent tunable bandgap, high carrier mobility,57 high current on/off ratio makes phosphorene an eligible candidate for FET-sensing applications. Mechanical cleavage and liquid exfoliation is the most commonly used method for the synthesis of phosphorene.58,59,60 Aptamer functionalized poly-L-lysine and black phosphorus (PLL-BP) nanosheet has been successfully designed to detect myoglobin.61 In addition, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (BP-PEDOT:PSS) BP nanocomposite has also been designed to detect hemoglobin.62 By the side of diagnostic, 2D black phosphorus also fnds application in the therapeutic feld. For example, it can be noted that BP nanosheets are widely used in photothermal therapy (PTT) and photodynamic therapy (PDT).63 In a study, Yang et al. developed BP-AuFe3O4 nanocomposite for combined PTT/PDT cancer therapy.64 Therefore, from the previous discussion, one can conclude that not only graphene or MoS2 but also other graphene-like nanomaterials possess certain properties that are well suited for biosensing as well as therapeutic applications.

11.3

CARDIOVASCULAR DISEASES (CVDs)

Cardiovascular disease is an umbrella term for a wide array of disease complications, commonly described as cerebrovascular disease, coronary heart disease, venous thromboembolism, peripheral artery disease, and rheumatoid and congenital heart diseases. About 31% of global deaths are caused by CVDs, making it one of the highly fatal complications among all classes of people worldwide.65 Apart from these, there are specifc cardiac diseases, which specifcally affect the heart muscles resulting in an ineffcient pumping of blood by the heart to

222

Emerging 2D Materials and Applications

other body parts, collectively called as cardiomyopathy. There are several types of cardiomyopathy, including dilated cardiomyopathy (DCM), hypertrophic cardiomyopathy (HCM), restrictive cardiomyopathy (RCM), arrhythmogenic cardiomyopathy (ACM), etc.20 For example, the occurrence of myocardial infarction (MI) among all these heart diseases has been signifcantly increasing every year. MI is the death of cardiomyocytes due to decreased blood fow in the heart followed by depletion of oxygen supply or ischemia. The predominant cause behind prolonged ischemia-induced cardiac tissue death is considered to be atherosclerosis or narrowing of blood vessel due to atheromatous plaques formation.66 The pathophysiology of MI starts with the accumulation of low-density lipoprotein (LDL) in the tunica intima layer of arteries, which results in the formation of oxidized-LDL (ox-LDL). This ox-LDL activates a cascade of infammatory cytokine and chemokine production and leads to the infltration of lymphocytes and monocytes. These events eventually form atherosclerotic plaque, and its subsequent rupture results in thrombosis, thus bringing the onset of MI.67 The major risk factors for atherosclerosis includes diabetes, hypertension, hyperlipidemia, smoking, in addition to gender and age.66

11.3.1

CARDIOVASCULAR BIOMARKERS AS DIAGNOSTIC PARAMETERS

There is a considerable number of cardiovascular biomarkers, which are assumed to complement the clinical tools meant for cardiovascular disease (CVD) symptom assessment, including electrocardiogram (ECG) and imaging techniques. These biomarkers aid in the evaluation of risk stratifcation, early prognosis, clinical diagnosis, as well as disease management of patients having cardiovascular complications, including acute myocardial infarction (AMI) and heart failure (HF)68 (Figure 11.1). An ideal biomarker needed for the early detection of myocardial injury requires to be predominantly expressed within the cardiac tissue exclusively at relatively higher levels, having a rapid release phenomenon after myocardial injury, having signifcantly higher clinical sensitivity, as well as specifcity in blood early after the onset of disease symptoms69 (Table 11.1). The most reliable cardiac biomarker, which is reported to be used for the early diagnosis of cardiac injury in CVDs especially in AMI is cardiac troponin (cTn). The cTn complex plays a crucial role in cardiomyocyte contraction by mediating the interaction between actin and myosin flaments. This complex is composed of three isoforms, including TnC (binds with calcium), TnI (blocks actin-myosin interaction), and TnT (binds with tropomyosin).70,69 A dynamic change equal to or higher than 20% in plasma concentration of cTn over 6–9 hours can be interpreted as an affrmation for AMI. Therefore, the elevated concentration of serum cardiac troponin (cTn) is still considered the gold standard for the early diagnosis of cardiac injury.71 Among all isoforms, cTnI is exclusively found in cardiomyocytes, hence the detection of cTnI is extremely specifc for diagnosing MI. Interestingly, cTn is secreted into blood after 6–8 hours of myocardial injury, gains its highest circulatory concentration after 12–24 hours, and remains there for seven to ten days. The only disadvantage is the diffcultly of predicting recurrent MI as cTn remains elevated in blood for a long time, thus prolonging its clearance.66

2D Nanomaterial(s)

Cardiac Biomarker/Risk Factor

Modifcations

1.

Graphene

Cardiac troponin I Porous graphene oxide (PrGO) is partially reduced and immobilized with the anticardiac troponin I (anti-cTnI) antibody on glassy carbon electrode (GC) (anti-cTnI/PrGO/GC) Cardiac troponin I Nitrogen-doped reduced graphene oxide (N-prGO) modifed with 1-pyrenecarboxylic acid (py-COOH) and poly(ethylene glycol) modifed pyrene (py-PEG) and integrated with Tro4 aptamer (Tro4 aptamer/ py-COOH/py-PEG /N-prGO/GC) Cardiac troponin I Amine functionalized graphene (f-GN) modifed with monoclonal anti-cTnI antibodies (Anti-cTnI/ f-GN) Cardiac troponin I 6-carboxyfuorescein (6-FAM) based anti-cTnI aptamers immobilized on the surface of graphene oxide (GO) sheet Cardiac troponin I Transition metal chalcogenide (nMo3Se4) embedded upon reduced graphene oxide (rGO) and functionalized by 3-aminopropyltriethoxy silane (APTES) and this composite further immobilized with monoclonal anti-cardiac troponin I (anti-cTnI) anti-cTnI /APTES/nMo3Se4-rGO/ITO BNP-32 Screen-printed electrodes (SPE) modifed by a polyethyleneimine (PEI)/reduced graphene oxide (rGO) nanocomposite and immobilized with A10 aptamer (A10 aptamer /SPE-rGO/PEI) BNP Reduced graphene oxide (rGO) fabricated on FET chip and subsequently decorated by platinum nanoparticles (PtNPs) D-dimer Graphene oxide (GO) was conjugated on surface plasmon resonance (SPR) chip surface and immobilized with anti-D-dimer. (anti-D-dimer/GO/SPR chip)

Detection Range/Lower Detection Limit Linearity range: 0.1–10 ng/ mL Detection limit: 0.07 ng/mL Detection limit:1 pg/mL

Application Used

References

EIS

22

DPV

98

Linear range: 0.01–1 ng/mL CV, LSV, EIS Detection limit: 0.01 ng/mL Linear range: 0.10–6.0 ng/mL FRET Detection limit: 0.07 ng/mL Linear range: 1 fg–100 ng/mL CV Detection limit: 1 fg/mL

83 99 100

Linear range:1 pg–1 μg/ mL Detection limit: 0.9 pg/mL

DPV

23

Detection limit: 100 fM/mL

FET

82

Linear range: 10–150 ng/mL Detection limit: 5.08 ng/mL

SPR

84

(Continued)

223

S.N.

2D Nanomaterials, Cardiovascular Diseases

TABLE 11.1 2D Nanomaterials Based Diagnostic Sensors for Cardiovascular Diseases

S.N.

2D Nanomaterial(s)

224

TABLE 11.1 (Continued) Cardiac Biomarker/Risk Factor Fibrinogen

MoS2

Anti-fbrinogen antibody attached with graphene (12HC-G) and deposited on screen printed electrodes (SPE) (12HC-G/SPE) Thrombin Thrombin binding aptamer (TBA) used graphene oxide as modifer and deposited on pencil graphite electrode (TBA/GO/PGE) Thrombin Graphene oxide (GO) sheet enriched with silver nanoparticles (AgNP) and modifed with thrombin binding aptamer (TBA2-thrombin-TBA1-AgNP-GO) CRP Anti-CRP antibody linked with reduced graphene oxide (rGO) and deposited on indium tin oxide (ITO) electrode (Anti-CRP/rGO/ITO) Cholesterol Protonation of the graphene carbon nitride sheet (g-C3N4H+) and doping with cylindrical spongy polypyrrole (CSPPy) and immobilized with cholesterol oxidase (ChOx) (ChOx-CSPPy-g-C3N4H+/GCE) PMP Graphene oxide combined with anti- αIIbβ3 antibody and deposited on glassy carbon electrode (Anti- αIIbβ3/GO/GCE) Nanocouple formulated using anti cTnT-labelled carbon dot (CD) Cardiac and molybdenum disulfde (MoS2) (anti cTnT-CD/MoS2) Troponin T Cardiac troponin I MoS2 nanosheets linked with cTnI based aptamer

Detection Range/Lower Detection Limit

Application Used

Linear range: 938–44542 mg/dL EIS Detection limit: ~ 500 mg/dL Linear range: 0.1–10 nM DPV Detection limit of 0.07 nM

References

85 101

Linear range: 0.05–5 nM/L Detection limit: 0.03 nmol/L

CV

86

Linear range: 1–10,000 ng/ mL Detection limit: 0.08 ng/mL Linear range: 0.02–5.0 mM detection limit: 8.0 μM

EIS

87

EIS, CV

Linear range: 100 to 7000/µL) EIS

Linear range: 0.1–50 ng/mL Detection limit: 0.12 ng/mL Linear range: 10 pM–1.0 μM Detection limit: 0.95 pM  Cardiac troponin I Copper nanowires/molybdenum disulfde/reduced graphene oxide Linear range: 5.0 × 10−13 to composite linked with cTnI based aptamer 1.0 × 10−10 g/mL (cTnI-aptamer/CuNWs/MoS2/rGO ) Detection limit: 1.0 × 10−13 g/mL Myoglobin Copper sulfde–molybdenum disulfde (CuS–MoS2) hybrid Linear range: 0.005–20 ng/mL nanostructures immobilized with anti-myoglobin detection antibody Detection limit: 1.2 pg/mL

102

103

FRET

89

EIS

90

EIS

91

CV

24

Emerging 2D Materials and Applications

2.

Modifcations

MMP9 Glucose

CRP

3. 3.1

Others Tungsten trioxide

3.2

3.4

Metal organic framework (MOF) 2D nickel MOF nanosheets Boron nitride

3.5

Black phosphorus

3.3

Tungsten trioxide nanosheets (WO3 NS) were functionalized with 3-aminopropyletriethoxysilane (APTES) and immobilized with cardiac troponin I antibodies (Anti-cTnI/APTES/WO3 NS/ITO) Cardiac troponin MOF functionalized with carboxyl-rich tris(4,4’-dicarboxylic I acid-2,2’-bipyridyl) ruthenium(II) (Ru(dcbpy)32+) h-FABP 2D nickel MOF nanosheets was fabricated PEI and luminol Troponin I

Myoglobin

Myoglobin

Thrombin

2D polyaniline layer

BNP

Deposition of AuNPs on boron nitride sheets and immobilization of thiol-functionalized DNA aptamer and deposited on fuorinedoped tin oxide (FTO) (Apt/AuNPs/BNNSs/FTO) Poly-l-lysine (PLL) deposited on black phosphorus nanosheets and facilitate the binding of anti-Mb DNA aptamers Fluorescent-dye-labeled single-strand DNA aptamer adsorbed on calcium-cation-doped polydopamine-modifed 2D black phosphorus nanosheets None

Fluorescence assay

75

Linear range: 53–854 pM Detection limit: 53 pM Linear range: 1 pM–10 nM Linear range: 10–300 mM Detection limit: 2.8 μM

EIS

92

FET EIS, CV

93 94

Detection limit: 8.38 fg/mL

(FET)

Linear range: 0.1–100 ng/mL

EIS

Linear range: 1 fg–10 ng/mL Detection limit: 0.48 fg/mL Linear range:100 fg–100 ng/mL Detection limit: 44.5 fg/mL Linear range: 0.1–100 µg/mL Detection limit: 34.6 ng/mL

ECL immunosensing ECL immunosensing CV, EIS, DPV

Linear range: 1 pg/mL–16 μg/ CV, EIS mL Detection limit: ∼0.524 pg/mL Linear range: 10–25 nM Fluorescence Detection limit: 0.02 nM assay Detection limit: 50 pg/mL

FET

104

97

105 106 96

61

95

107

225

3.6

MoS2 feld-effect transistor (FET) functionalized with Amyloid β1–42 Glassy carbon electrode modifed with molybdenum disulfde (MoS2) nanosheets decorated with gold nanoparticles (AuNPs) and immobilized with glucose oxidase (GOxAuNPs@MoS2/GCE) MoS2 and gold nanoparticles fabricated with black gated feld-effect transistor (Au-NPs/MoS2 BG-FET)

Detection limit: ~100 fM for all three microRNAs

2D Nanomaterials, Cardiovascular Diseases

microRNA-499, Duplex specifc nuclease and molybdenum disulfde (DSNmicroRNA-133a, MoS2) conjugated with aptamer and microRNA-1. Thrombin MoS2 nanosheets conjugated with ssDNA aptamer

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Apart from cTn, creatine kinase (CK), a cellular energy metabolism regulator as well as its isoform, i.e., CK-myocardial band (CK-MB), is also considered as a gold standard for the detection of cardiac injury. It has been reported that CK-MB constitutes about 30% of CK in the myocardium, thus making this enzyme a crucial biomarker of myocardial injury. Moreover, a rise beyond 5% of total CK activity in blood suggests possible damage to the heart muscle. In addition, CK-MB appears in circulation 4–6 hours after the onset of myocardial injury, gains its highest circulatory concentration after 10–12 hours, and remains there for about two days.72,73 This biomarker has about 91% sensitivity as well as specifcity for the disease diagnosis for which it has become widely used in clinical emergency departments. The expression of CK as well as CK-MB is not restricted to the heart only, thus reducing their diagnostic specifcity for AMI.71 Myoglobin (Mb) is an iron-binding protein located primarily in the cardiac and skeletal muscle tissues, where it carries oxygen to muscle cells. It is a very sensitive biomarker for the prediction of muscle injury, especially in myocardial injury. Mb appears in circulation 1 hour after myocardial damage, gains its highest circulatory concentration after 8–10 hours, and remains there for about 24 hours. The major disadvantage of this biomarker is its poor specifcity despite being useful in evaluating reperfusion as well as infarct size in AMI.73,72 Recent research has supplemented a few novel sets of biomarkers for the diagnosis of myocardial necrosis, including sarcomeric cardiac myosin-binding protein C (cMyC), ischemia-modifed albumin (IMA), and heart-type fatty acid–binding protein (hFABP).71 The h-FABP is found predominantly in cardiomyocytes, where it helps in fatty acid transport and metabolism. Furthermore, recent studies have confrmed that h-FABP is rapidly secreted into the cytoplasm upon MI, thus making this protein an emerging potential biomarker for MI.74,70 Similarly, cMyC is a myosinassociated protein, which is exclusively expressed by cardiomyocytes, thus making it a specifc biomarker for myocardial injury.71 IMA is assumed to be N-terminalmodifed albumin having reduced cobalt binding affnity, where its level is attributed upon myocardial injury, thus making this protein a potential biomarker for MI.73 Recent studies have also confrmed that there are certain miRNAs that can predict the potential future onset of CVD. For instance, miR-21, miR-27b, miR-126, miR130a, miR-221, miR-222, and let-7 are involved in the patho-progression of angiogenesis, atherosclerosis, congenital heart defects, and coronary artery disease.68 In addition, miRNA-499, miRNA-133a, and miRNA-1 are also considered to be the reliable biomarkers for MI.75 Besides these, several other potential biomarkers are often detected during CVDs, especially MI and atherosclerosis, including N-terminal pro-B-type natriuretic peptide (NT-proBNP), mid-regional pro-adrenomedullin (MR-proADM), midregional pro-atrial natriuretic peptide (MR-proANP), C-terminal pro-endothelin 1 (CT-proET-1), high sensitivity C-reactive protein (hs-CRP), low-density lipoprotein (LDL), pregnancy-associated plasma protein-A (PAPP-A), myeloperoxidase (MPO), and copeptin.66,69,70,71 In addition to cardiac biomarkers, cardiac risk factors also play signifcant roles in the assessment of cardiovascular disease. Cardiac risk factors are described as a person’s risk for the development of cardiac disease in the near future. High level of

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measurable cardiac risk factors such as glucose,76 thrombin,77 fbrinogen,78 cholesterol,79 etc. help to predict the prognosis of diseases.

11.4 2D NANOMATERIALS IN DIAGNOSTICS As it has been noted previously, increased concentration of cardiac biomarkers and risk factors played a signifcant role in the early diagnosis of cardiovascular complications.66,69,70 In recent years, biosensing emerges as the most promising area in diagnostic aspects of cardiac markers.23 Electrochemical biosensing is most widely used in this area, where an immobilized recognition element selectively binds with an analyte, and this biding is transformed to an electrical signal, which is seen as a digital imprint on the connected computer screen. Based on their transducer, biosensors are usually classifed into potentiometric, amperometric, and impedimetric. The main methodologies implied in these biosensors are electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), linear sweep voltammetry (LSV), and feld-effect transistors (FET) based biosensing.80,80 The translation of these devices into clinical practices allows the 2D nanomaterials based sensors to be employed as a diagnostic tool for cardiac biomarkers. Therefore, this section deals with the diagnostic aspects of 2D nanomaterials in cardiac diseases (Table 11.1).

11.4.1

GRAPHENE

Graphene has been preferred as a 2D nanomaterial to be used in a biosensor due to its robust mechanical strength, high surface area, and better electrical conductivity capacity. The graphene and graphene-modifed biosensors detect several cardiac biomarkers, including myoglobin, cTn, and BNP, which are supposed to be released upon cardiac injury.25 In a recent experiment, Singh et al. designed a screen-printed electrode (SPE) coated with a composite consisting of gold nanoparticles and reduced graphene oxide (AuNPs-rGO) to be used as an immunosensor for the detection of cardiac myocardial myoglobin. The immunosensor displayed a dynamic range of detection of myocardial myoglobin, i.e., 1–1400 ng/ mL with a signifcantly better detection limit of ~0.67 ng/mL in contrast to the detection limit of ELISA (~4 ng/mL).81 In a similar study, Kazemi et al. designed a nano-immunosensor (cTnI/PrGO/GC) by coating a glassy carbon (GC) electrode with porous GO (prGO) and further functionalized with cTnI antibodies for the serum detection of human cTnI. This nano-immunosensor had displayed an outstanding sensitivity as well as selectivity for the detection of myocardial cTnI in the dynamic range. i.e., 0.1–10 ng/mL detection limit of ~0.07 ng/mL.22 In another study, Lei et al. demonstrated that reduced graphene oxide (rGO) feldeffect transistor (FET) chip decorated with platinum nanoparticles (PtNPs) and further immobilized with BNP antibody can be an effective biosensor for the detection of BNP in human blood. This designed biosensor can detect blood BNP at as low as 100 fM, thus making it an effective one with high sensitivity and specifcity for the prediction of HF and CVDs.82 Tuteja et al. demonstrated amine functionalized graphene (f-GN) for the detection of cardiac troponin I with the

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linearity range of 0.01–1 ng/mL.83 The graphene-based biosensor also implied to detect other biomarkers or risk factors such as BNP,23,82 D-dimer,84 fbrinogen,85 thrombin,86 CRP,87 etc. (Table 11.1).

11.4.2

MOS2

MoS2 is the second-most studied 2D nanomaterial after graphene. Its unique optical, physiochemical, and biological properties presented its wide potential in diagnostic tools. Typically, MoS2 is used in industries such as dry lubricants and negative electrode material in lithium-ion batteries, but its 2D form implies its potential in the biomedical feld such as sensing, therapy, as well as imaging.88 Accordingly, MoS2 is implied in the diagnostics tools for cardiac biomarker and risk factor detection. Recently, Gogoi et al. demonstrated a Förster resonance energy transfer (FRET) technique to detect the cardiac troponin T (cTnT), which is based on the MoS2 nanosheet combined with anti-cTnT labelled carbon dots (CDs). The FRET-based sensor displayed a linearity range of 0.1–50 ng/mL and implied a lower detection limit of 0.12 ng/mL.89 Similarly, Qiao et al. used electrochemical impedance spectroscopy (EIS) and displayed aptamer-based cardiac troponin I biosensor with the usage of MoS2 nanosheets as matrix material. It exhibits a linear detection range of 10 pM to 1.0 μM/mL with a lower detection limit of 0.95 pM/mL.90 Another cardiac troponin I detection biomarker used aptamer/CuNWs/MoS2/rGO composite and its linearity displayed in 5.0  ×  10 −13  to 1.0  ×  10 −10  g/mL, having a detection limit of 1.0 × 10 −13  g/mL.91 Zhang et al. used hybrid nanostructures (CuS-MoS2) along with monoclonal mouse antihuman myoglobin capture antibody for myoglobin detection. This immune complex had a relative response with myoglobin concentration in the range of 0.005–20 ng/mL, with a detection limit of 1.2 pg/mL.24 MoS2 has also been used in the multiplexing cardiac biomarker biosensor such as Zhu et al. detected microRNA-499, microRNA-133a, and microRNA-1 via fuorescence assay having lower detection limits around 100 fM for all three microRNAs.75 Additional cardiac marker and risk factors including thrombin,92 MMP9,93 glucose,94 CRP94 have also been detected via MoS2 based biosensing techniques.

11.4.3

OTHER 2D NANOMATERIALS

Apart from graphene and MoS2, other 2D nanomaterials also exhibit their diagnostic applications in cardiovascular diseases. Kumar et al. used black phosphorus nanosheets and functionalized them with poly-l-lysine (PLL) for facilitative binding of anti-Mb DNA aptamers for the detection of myoglobin. The highly sensitive developed biosensor displayed a dynamic response range from 1 pg/mL to 16 μg/ mL with the detection limit of ∼0.524 pg/mL.61 Recently, black phosphorus has also been modifed with calcium-cation-doped polydopamine (PDA) for the detection of thrombin.95 In another fnding, Adeel et al. developed an electrochemical aptasensor using boron nitride nanosheets decorated with gold nanoparticles for the detection of myoglobin.96 Another 2D nanomaterial, that is, tungsten trioxide nanosheets (WO3NS) has also been used to develop biosensor for troponin I detection with linearity response of 0.1 to 100 ng/mL.97

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11.5

THERAPEUTIC APPLICATION OF 2D NANOMATERIALS

Cardiac injury can lead to the death of functional cardiomyocytes, which cannot be regenerated by the heart itself, thus generating a permanent scar at the injury site. It can be detrimental to the healthy pumping function of a heart. If large enough, it can lead to heart failure also. Tissue engineering can provide a solution by blending cells and nanomaterials to construct biocompatible scaffolds and injectable hydrogels in order to replace and/or regenerate the lost or dead cardiomyocytes after cardiac injury, especially MI. Among all nanomaterials, graphene and its modifed forms signifcantly supplement the biocompatible scaffolds for cardiac tissue engineering25 (Table 11.2). Recently, Saravanan et al. demonstrated GO-Au nanosheets TABLE 11.2 Role of 2D Nanomaterials in Cardiac Tissue Engineering

S.N. 1

2D Nanomaterial(s) Graphene

Modifcations rGO-GelMA (Gelatin Methacrylyol) Hydrogel

GO-Col (Collagen) Scaffold Matrix rGO-Ag-PU (Polyurethane) Matrix

GO-CS (Chitosan Scaffold) Matrix GO-Au-CS (Chitosan Scaffold) Matrix

Properties Enhanced electrical conductivity and mechanical properties

Applications in Tissue Engineering

Promotes cell viability, proliferation, and maturation in cardiomyocytes. These cardiomyocytes exhibit stronger contractility and faster spontaneous beating rate on rGO-GelMA hydrogel. Shows enhanced Promotes neonatal electrical signal cardiomyocyte adhesion and propagation upregulates the expression of among cardiac the cardiac specifc genes like cells Cx43, Actin4, and Trpt-2. Shows enhanced Promotes cell adhesion, electrical and increases cytocompatibility, mechanical upregulates the expression of properties cardiac specifc genes, including GATA-4, TBX-18, cTnI, and α-MHC in hCPCs. Shows good Promotes cell adhesion, swelling, porosity, intercellular networking, and conductive and cell viability in in properties H9C2 rat cardiomyoblasts. Shows excellent Promotes cell adhesion in porous rat SMCs and mouse architecture, fbroblasts and potentiates improved swelling growth with minimal capacity, increased cytotoxicity in a rat model conductivity and of MI. biodegradability

References 116

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118

109

108

(Continued )

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TABLE 11.2 (Continued)

S.N.

2D Nanomaterial(s)

Modifcations GO-OPF (oligo (poly (ethylene glycol) fumarate)) Hydrogel

GO-Alginate Microgel

2

Molybdenum Disulfde (MoS2)

GO- FA-PEG (Polyethylenimine- Folic Acid) Matrix MoS2 Nanoparticles

MoS2-Nylon Nanofber Scaffold

MoS2-SF (Silk Fibroin) Nanofber Scaffold

Properties Shows enhanced semiconductive properties that were lacking in pure OPF

Applications in Tissue Engineering

Promotes cell adhesion of rat cardiac fbroblasts, generation of cytoskeletal structure, Ca2+ signal conduction, and intercalated disc assembly in cardiomyocytes of rat model of MI. Shows good Promotes cell viability of swelling, porosity, hMSCs and enhances and conductive cardiac maturation in an properties oxidative stress condition in rat model of MI. Shows better Mitigates ROS in aqueous stability macrophages and promotes than individual cardiac tissue repair by GO-PEG reducing fbrosis in mouse model of MI. Shows good Promotes the proliferation electrical of H9C2 cardiomyocytes, conductivity, and this nanomaterial excellent protects H9C2 cells from biocompatibility, oxidative stress induced by and high surface H2O2. area/mass ratio Shows enhanced Promotes cellular mechanical attachment and cell properties and proliferation along with electrical upregulating cardiac conductivity functional genes, including GATA-4, c-TnT, Nkx 2.5 and α-MHC in mECCs. Shows enhanced Promotes growth and mechanical differentiation of properties, elastic TBX18-hiPSCs into cardiac properties, and cells. It also enhances cell electrical attachment and viability by conductivity upregulation of cardiac functional genes, including GATA-4, c-TnT, and α-MHC.

References 110

111

113

115

114

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in CS scaffold to be an excellent porous biomaterial having controlled degradation properties along with the desired swelling capacity for cardiac tissue engineering. In addition, it not only exhibited a better electrical conductivity (improved QRS interval) but also supported cell adhesion and growth with minimal cytotoxicity in a rat model of MI.108 In a similar study, Jiang et al. fabricated a Chitosan Scaffold (CS) by blending with GO, which displayed an excellent biomaterial for promoting cell adhesion, intercellular networking, as well as cell viability by upregulating the expression of cardiac-specifc proteins, like connexin 43 in H9C2 cells.109 In another study, Zhou et al. developed a novel conductive hydrogel by introducing GO into an oligo (poly(ethylene glycol) fumarate) (OPF) hydrogel. This OPF/GO hydrogel proved to improve cell adhesion of rat cardiac fbroblasts in vitro accompanied by the generation of cytoskeletal structure, Ca2+ signal conduction, and intercalated disc assembly in cardiomyocytes of rat model of MI.110 In a recent study, Choe et al. demonstrated that GO/alginate microgel could be a potential encapsulated carrier for stem cells to initiate cardiac tissue regeneration. The group co-cultured the cardiomyocytes (CMs) with human mesenchymal stem cell (hMSCs) encapsulated with GO/alginate microgel and showed the increased potential of cell viability and cardiac maturation in an oxidative stress condition in the rat model of MI. This fnding could provide an effective platform for stem cell therapeutic regeneration in cardiac diseases, including MI.111 In a similar study, Zhao et al. developed a gel matrix (GO-RTG) by modifying reverse thermal gel (RTG) with GOs, which could enhance the growth, proliferation, and viability of neonatal rat ventricular myocytes (NRMs), thus could potentially be a therapeutic candidate for tissue regeneration after cardiac injury.112 In a novel study, Han et al. formulated a macrophage-targeting/polarizing GO complex (MGC) by modifying GO with polyethylenimine (PEI) and folic acid-PEG (FA-PEG). Furthermore, the MGC decreased ROS in macrophages and induced polarization of M1 macrophage (secrets pro-infammatory cytokines) into M2 macrophage (secrets anti-infammatory cytokines), thus reducing fbrosis and improving cardiac tissue repair in a mouse model of MI113 (Table 11.2). Molybdenum disulfde (MoS2) is a transition metal dichalcogenide that functions as a new generation 2D nanosheet having excellent properties like good electrical conductivity, excellent biocompatibility, and high surface area/mass ratio. New studies are providing emerging evidences for MoS2 to be used in cardiac tissue engineering.114 For example, Ming et al. demonstrated that MoS2 nanoparticles could promote the growth and proliferation of H9C2 cardiomyocytes apart from protecting these cells from H2O2-induced ROS mediated oxidative stress. This fnding suggests that MoS2 can be used as a therapeutic option for cardiomyocyte proliferation during the repair of cardiac injury.115 In another study, Nazari et al. designed a nanofber scaffold, MoS2-Nylon6 by conjugating MoS2 nanosheets into nylon6 electrospun nanofbers, which displayed enhanced mechanical properties and electrical conductivity. This MoS2-Nylon6 scaffold promoted cellular attachment and cell proliferation along with upregulating cardiac functional genes, including GATA-4, c-TnT, Nkx 2.5, and α-MHC in mouse embryonic cardiac cells (mECCs).114 In a similar study, Nazari et al. incorporated MoS2 into silk fbroin (SF) nanofbers and demonstrated that MoS2-SF potentially promoted growth and differentiation of TBX18hiPSCs (human-induced pluripotent stem cells) into cardiac cells. It also enhances

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cell attachment and viability by upregulation of cardiac functional genes, including GATA-4, c-TnT, and α-MHC.115 Hence both GO as well as MoS2 are being implemented in cardiac tissue engineering to regenerate or repair the lost cells during cardiac injury (Table 11.2).

11.6

CHALLENGES AND FUTURE PERSPECTIVE

2D nanomaterials are an emerging group of biomaterials having unique physical, chemical, optical, thermal, as well as electronic properties, making them remarkable potential candidates for diagnostic applications (e.g., biosensing and bioimaging), therapeutic applications (e.g., drug delivery), and regenerative medicines (e.g., tissue engineering) (Figure 11.1). As in this chapter, we had earlier discussed in detail about the need of therapeutic alternative(s) in cardiac complications like MI. The 2D nanomaterials have exhibited high effciency in biosensing the cardiac biomarkers in CVDs, especially in MI, signifying their propitious utilization in biomedical practices (Table 11.1). The application of 2D nanomaterials in biosensors has been a promising avenue for the health care industry due to their excellent electrical conductivity and planar nature. The major challenge in the fabrication of biosensors is a lowcost yet controllable synthesis route (management of surface charge density), which hinders in its implementation for mass production. Furthermore, the biocompatibility and environmental stability both in storage condition and testing medium remain challenging for real-life application of 2D nanomaterial-based biosensors.8 Apart from the use in diagnostic biosensors, the advancement of 2D nanomaterials in the fabrication of biocompatible cardiac scaffolds has been commendable (Table 11.2). Although these 2D nanomaterial-based scaffolds are still in experimental phases owing to their cytotoxic effects,120,121 which need to be optimized before being effectively applied in biomedical practices of cardiac tissue engineering. There are several challenges that need to be overcome, including the reproducibility of 2D nanomaterial behavior in animal models, along with their molecular interaction with other cellular and matrix biomolecules. Apart from these, the biodegradability of these 2D nanomaterials is a considerable threat in in vivo experiments, where they are prone to cause cytotoxic effects in a prolonged condition.14 As we move from basic biotechnology toward translational theranostics, innovating strategies will emerge to develop reliable, safe, and highly effcient 2D nanomaterials for interdisciplinary research and biomedical applications.

11.7 CONCLUSIONS 2D nanomaterials are ultrathin nanomaterials that display extremely high degree of anisotropy in addition to good biochemical functionality. These 2D nanomaterials include carbon-based nanosheets like GO nanosheets, TMDs, MoS2 nanosheets, boron nitride nanosheet, and metal-organic framework (MOF) nanosheets. Over the past few years, a variety of 2D nanomaterials have emerged as potential candidates for the restructuring, repair, and regeneration of damaged and/or injured tissues and organs (Figure 11.1). The use of 2D nanomaterials has dramatically revolutionized biomedical felds, especially for bone and skin repair, neural regeneration, wound

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healing, and cardiac tissue engineering. Among these, cardiac tissue injury has remained challenging due to the intrinsic inability of the damaged cardiac tissue to regenerate after an injury like stroke and MI. Hence, diagnostic tools are vital for determination of these complications. 2D nanomaterials have been successful in the construction of electrochemical biosensors for the detection of cardiac biomarkers after myocardial injury, especially in MI, owing to their high electrical conductivity and large surface area (Table 11.1). Apart from this, tissue engineering has also emerged as an effective tool for restructuring, repair, and regeneration of damaged and/or injured tissues and organs. In tissue engineering, biocompatible scaffolds are pivotal as they provide structural support for tissue repair along with serving as a template for tissue restructuring and regeneration. The construction of biocompatible scaffolds are possible due to their unique physiochemical properties like uniform shapes, good surface charge, high surface-to-volume ratio, and superior porosity, which enable these nanomaterials to be used in the feld cardiac tissue engineering (Table 11.2). Although the potential application of 2D based nanomaterials in cardiac tissue engineering in still in its infancy, future research will elucidate these biomedical applications.

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Index A AAS, 201 absorption, 48, 147 absorption coeffcient, 145, 147 adatoms, 24 AFM, 200 Ag, 201 allotropes, 198 ambient environment, 52 aminoglycosides, macrolides, b-lactams, vancomycin, fuoroquinolone, phenols, 196 anisotropic, 44, 48 anisotropy easy axis anisotropy, 104 magnetic anisotropy, 95, 96, 101, 112 magnetocrystalline anisotropy, 102 perpendicular anisotropy, 102 shape anisotropy, 96 single ion anisotropy, 104 surface anisotropy, 96 anthropogenic, 195 antibiotics, 196, 204 antiferromagnetic, 96–99, 101–104 Ising-type antiferromagnet, 104 antigens, 81 aqueous, 45 arsenic, 196 asthma, 196 Au, 201 Au Nps, 201

B bacteria Salmonella typhimurium, Escherichia coli, 206 Staphylococcus aureus, Listeria monocytogenes, 206 band gap, 43, 44, 46, 49, 71 direct band gap, 97, 98 indirect band gap, 97, 98 band gap tunability, 145–147, 158 band structure, 9 bathocuproine, 151 batteries, 9, 157, 158 bilayer graphene, 25 biomedical, 80 bipolar, 41, 50 black phosphorus (BP), 73, 132–133, 170, 182, 198, 221 boron nitride, 220

boron nitride nanosheet, 197 bottom-up, 10 Brillouin zone, 20 broadband, 71 buffer layer, 148, 149, 151 bulk material, 7

C cadmium (Cd), 196, 202 carbamates, 196 carbide, 45, 133–134, 157, 171, 179, 200 carbon, 40, 41, 45, 46 carbon dioxide (CO2), 196, 198 carbon monoxide (CO), 196 carbonitrides, 12 carbon paste, 149 carcinogenic, 196, 201 cardiac myosin-binding protein C, 226 cardiac troponin, 222 cardiovascular disease, 221, 222 catacol, 203 catalytic effect, 8 chalcogenide(s), 8, 145–147, 150, 154, 156 charge, 40, 41, 48, 49 charge carrier, 49, 51 charge transport, 51, 58 chemical potential, 30 chemical vapor deposition (CVD), 11, 98, 107–108 chemilumiscence, 197 chromium, 200, 201 CNT, 18 Co, 201 coherent-potential approximation, 28 conductive polymers, 153, 154 copper (Cu), 196 counter electrode, 149–151, 155, 156 Cr2Ge2Te6, 101, 107, 108 creatine kinase, 226 CrI3, 98–102, 107 Curie temperature, 98, 101–102, 104 cyclic voltammetry (CV), 200

D dangling, 49 defect chemisorbed defect, 96 defect engineering, 107 point defect, 96 vacancy defect, 96

241

242 demagnetization feld, 96 diamagnetism diamagnetic, 97, 98 dichalcogenides, 146–148, 150, 151, 157, 158 diffusion length, 146, 147 diluted magnetic semiconductor (DMS), 98 diode, 46, 50 dipole moment, 156 dirac points, 20 DMSO, 155 DNA, 201 DNA hybridization, 81 DNT, 202 domains, 97, 101, 110, 111 doping, 72, 98, 101–103, 105, 107 double-walled carbon nanotubes, 24 DPV, 201 dye-sensitized solar cells, 155

E eco-system, 196 effciency, 145, 146, 149–152, 155, 157 EIS, 200 electrical, 41, 43, 46, 47 electrolyte, 155, 158 electromechanical, 145 electron, 44, 47, 50 electronics, 196 electron transport layer, 147–151, 155 element, 45 ELISA, 206 energy-harvesting, 145–147 energy states dispersed edge states (DESs), 97 zero-energy states (ZESs), 97 energy storage, 157–158 enzymes, 81 esophagus, 196 EtOH, 209 exchange interactions, 95, 101, 104–105, 107 excitons exciton emission, 104 exciton-magnon coupling, 104, 105 exciton-phonon coupling, 105 exfoliation, 101, 107, 112 chemical exfoliation, 108 liquid phase exfoliation (LPE), 107–108 mechanical exfoliation, 107 extrinsic defects, 24

F Fe3GeTe3, 102–103 ferromagnetic (FM), 96–98, 101–102, 104, 107, 110–113 FET, 21

Index fber, 48 feld-effect transistor (FETs), 2 fgure of merit, 152–155 fexibility, 147, 151, 157 fexible, 75 fexible transistor, 9 fuorescence, 197 fuorometry, 209 forest fre, 196 fossil fuels, 145 fuel cells, 157, 158, 183 fullerene, 18 functional, 7 furazolidone, 205

G gas chromatography (GC), 204 gas monitoring, 197 GC-MS, 208 germanene, 8 global warming, 196 grain boundaries, 98 graphene, 1, 7, 17, 73, 196, 197, 145–151, 155, 157, 158, 167 graphene nano ribbon, 24 graphene quantum dots (GQDs), 97, 148, 149 graphite, 8, 124–127 graphitic carbon nitride, 221 green revolution, 196

H Hall effect, 111 anomalous Hall effect, 102 anomalous Hall resistance, 102 Hall effect sensors, 111 Hooge’s parameter, 111 quantum anomalous Hall (QAH) effect, 105 quantum spin Hall (QSH) effect, 105 h-BN, 157, 168 heavy metals, 195, 196, 200, 201 Heisenberg Hamiltonian anisotropic Heisenberg Hamiltonian, 104 isotropic Heisenberg Hamiltonian, 104 heterostructure, 47, 49, 76 hexagonal, 45, 54 hexagonal boron nitride, 21, 130–132 high-performance liquid chromatography (HPLC), 204 hole transport layer, 147, 149, 151, 155 honey comb, 198 honeycomb lattice structure, 103–104 hybrid energy storage (HES) device, 183 hydrazine, 13 hydrogen sulfde (H2S), 196

243

Index hydroquinone, 203 hydrothermal, 11 hydrothermal method, 151

I image sensor, 76 indirect band structure, 154 inductive coupled plasma mass spectroscopy, 201 interface, 148, 149 interfacial layers, 148, 158 interlayer excitons, 86 intrinsic defect, 24 ion intercalation, 11 iron (Fe), 196

J Jahn-Teller distortions, 98

K Kelvin probe force microscopy, 23 kidney, 196, 201 Klein tunneling, 20 Kosterlitz-Thouless (KT) transition, 104

L layered thin flms, 158 LC-MS, 204 lead (Pb), 196, 201, 202 light-emitting diodes (LEDs), 83 li-ion batteries, 173 linear dichroism (LD), 105 liver, 196 logic gates, 76 lung, 196 lung cancer, 196, 201 lymphocyte cells, 81

M magnetic anisotropy energy (MAE), 101 magnetic circular dichroism (MCD), 98, 109 magnetic force microscopy (MFM), 97, 109–110 magnetic gap, 107 magnetic hysteresis, 98, 100–102, 107 magnetic moment, 96–97, 105, 107 magnetic tunnel junction (MTJ), 112–113 tunnel barrier, 112 magnetocrystalline anisotropy, 102 magneto-optical Kerr effect (MOKE), 98, 102, 109–110

circular birefringence, 110 Kerr rotation, 110 magnetoresistance (MR), 98–99, 101, 105, 107, 111–112 magnetostatic energy, 95, 96 magnon excitations, 102, 104 magnon excitation gap, 104 mass spectrometry (MS), 204 MAX phase, 13, 87, 133, 136 mechanical exfoliation, 10 mechanical strength, 8 mechanochemical exfoliation, 12 memory, 75 MeOH, 209 mercury (Hg), 196, 201, 202 Mermin-Wagner theorem, 96 metal carbides, 12 metal organic framework (MOF), 157, 199 metal oxides, 157 methane (CH4), 196, 207 metronidazole, 205 mica, 136 microorganism, 196 microwave, 11 midecamycin, 205 minamata, 201 MO3, 199 mobility, 34, 98, 111 moiré patterns, 137 molecular beam epitaxy (MBE), 102, 107–108 molybdenum disulfde (MoS2), 219, 228, 197, 198, 201, 208 monolayers, 146 MoS2, 2, 97–99, 8, 128–130 multilayer graphene, 25, 30 MXenes, 1, 3, 4, 8, 133–136, 146, 158, 171, 180, 200 myoglobin, 226

N NAAT, 206 nanogenerators, 157 nanomaterials, 13 nanoparticles, 83 nanoscale, 7 nanosheets, 7 nanowire, 7 natriuretic peptide, 226 Néel temperature, 104 neuromorphic technology, 83 neutron scattering, 104 NiPS3, 104–105 nitrides, 12, 146 nitrobenzene, 203 nitrogen dioxide (NO2), 198, 207 nitromethane, 203 non-biodegradable, 201

244 nonrenewable, 145 non-volatile, 76

O open circuit voltage, 148 optical, 9, 196 optical window, 150 optoelectronic properties, 148, 158 optoelectronics, 71 organochlorine, 196 organophosphates, 196 oxide thermoelectric, 152 oxygen vacancies, 150 ozone degradation, 196

P paramagnetism, 97, 110 paraoxon ethyl (PXL), 204 Pd Nps, 204 peizoresistive, 9 perovskite solar cells, 145–149 pesticides carbamate, organophosphate, organochlorine, pyrethrin, pyrethroid, 196, 203 phenol, 203 phenolic compound 2-CP, 4-CP, 202 phosphorene, 170 photocatalyst, 158 photoconductance, 71 photodetector, 9, 71, 72 photoelectric effect, 71 photoinduced, 158 photoluminescence, 13, 71 photonics, 196 photoresponsivity, 72 photovoltaic, 73 piezoelectric, 146, 147, 152, 156, 158 plasma mass spectrometry, 197 plasmonic, 81 P-N junction, 73 poisonous, 196 polarized neutron refectometry (PNR), 107 pollutant, 196, 200 power conversion effciency, 146, 149, 150, 151, 155 power density, 73 precursor, 12 prostate, 196 proteins, 81 proteinuria, 201 Prussian blue, 204 pulse laser deposition, 12, 153 pyrolytic graphite, 10

Index Q quantum confnement, 9, 152, 154 quantum dot, 7 quasi-electron scattering (QES), 104

R Raman spectroscopy, 104, 109 recombination process, 71 refection high-energy electron diffraction (RHEED), 108 renewable energy, 73

S Seebeck coeffcient, 153, 154 SEM, 200 semiconductors, 71 sensor(s), 9, 196 FET, 202, 207 gas, 198 SERS, 204 silicene, 8, 136 single-layer graphene, 20 skin, 196 Sn3O4, 205 SnO2, 199 sodium ion batteries, 176 soft chemical synthesis, 11 solar energy, 73 solvothermal, 11 source, 196 specifc capacitance, 157 specifc capacity, 10 spin diffusion length, 96, 111 spin feld effect transistor, 111 Rashba spin-orbit interaction, 111 spin lifetime anisotropy effect (SLA), 112 spin precession, 112 spin-orbit coupling, 98, 105, 112 spin polarization, 98, 112 spin-polarized currents, 105 spin-valve, 98–99, 111 spin diffusion length, 96, 111 spin fltering effects, 111 spin-injection effciency, 111 spin-wave excitation gap, 96 Spiro- OMeTAD, 147 spray pyrolysis, 151, 153 STEM, 200 stomach, 196 Stone-Wales defect, 24 sunset yellow, 203 supercapacitors, 9, 157, 158, 179

245

Index superconducting quantum interference device magnetometer (SQUID), 98, 107, 109–110 Josephson junction, 110 magnetic fux, 110 superexchange mechanism, 101 super-paramagnetism, 96 surface-to-volume ratio, 10 SWASV, 200 SWV, 200 synapse, 83

twisted bilayer graphene, 27 two dimensional materials, 1, 7

T

van der Waals, 11, 49, 56, 59, 172 van der Waals (vdW) forces, 97, 103–104, 107 van der Waals semiconductors, 101 vibrating-sample magnetometer (VSM), 109 volatile organic compound (VOC) xylene, toluene, acetone, ethanol, isopropanol, glucose, formaldehyde, 195, 208 volcanic eruption, 196

TEM, 200 ternary transition metal chalcogenides and halides (TMCHs), 101 tetracyclines, 196 tetrazine, 203 TG, 200 Th, 201 thermal fuctuations, 96 thermal stability, 146, 153, 158 thermocouple, 154 thermoelectric, 146, 147, 152–155, 158 thiocarbamates, 196 threshold, 83 time-reversal symmetry (TRS), 105 TiSi2, 207 TLC, 204 TNT, 202 top-down approach, 10, 107 topological defect, 25 topological insulators, 105, 108 toxic gases, 195 transition metal dichalcogenides (TMDs), 1, 97, 146, 147, 150, 151, 157, 158, 169, 181, 198, 220 transition metal halides (TMHs), 98 transition metal phosphorous trichalcogenides (TMPCs), 103, 127–130 trapping, 76 tunability, 145–148, 151 tunnel magnetoresistance (TMR), 101

U ultra-sonication, 10 unsaturated bonds, 8 UV-Vis spectroscopy, 200, 201

V

W waste treatment, 196 wearable technology, 89 wearable thermoelectric, 154 white graphene, 168 WO3, 199 WSe2, 209

X XPS, 200 X-ray fuorescence, 201 X-ray photoelectron spectroscopy (XPS), 98

Z zinc (Zn), 196, 201 zinc air batteries, 175 zinc ion batteries, 174 ZnO, 199, 207 ZrO2, 204